By John Evans

Special to Yahoo Sports

It's never a great idea to be rigid in our opinions when it comes to fantasy football. Things change pretty fast in the NFL, and now we have clues from training camp and the preseason to sift through. Injuries, team performance, and player usage in August can present a very different picture than July’s. I pay particular attention to a team’s offensive line and how that unit may stack up against defenses — it can have a big impact on our fantasy assets!

The player I’ve changed my mind the most about is an RB who wears the silver and Honolulu blue of the Motor City …

Kerryon Johnson, Detroit Lions

I was reticent to endorse Johnson this year — not because I doubt his talent or even that of his offensive line, but because of reports that Matt Patricia will emulate the committee backfield used by his old team. Everyone has a role on the Patriots, and if the offense is a world-beater, that’s fine. Detroit’s may not be. It may even be in the bottom half of the league, so getting one-third of a committee in a subpar offense isn’t a recipe for fantasy feasting.

First off, the Lions cutting ties with receiving back Theo Riddick brightened Johnson’s outlook considerably. If he’s playing on passing downs, that gives him more chances to gain yards and tally receptions (cheap points!). Yes, sixth-round pick Ty Johnson is a breakaway threat who should be in the mix, but I doubt that Patricia will entrust the rookie with regular pass-protection responsibility. Some are penciling him in for a third-down role, but at Maryland, he was subbed out in passing situations. That’s telling.

While Ty is not a threat to Kerryon’s snaps, C.J. Anderson is. The squatty back certainly has the power to secure a short-yardage role. But while Anderson’s pass protection can be relied upon, he’s averaged 1.5 catches and 13 receiving yards in his 69 career games. Johnson basically doubled those averages as a rookie and, at Auburn, had one drop on 56 catchable targets. Not only is he effective on screens, swings, and check-downs, he graded well as a rookie in pass pro. I’d say 50 catches is his floor this year.

While Anderson’s preseason usage suggests he’ll prevent the second-year stud from monster workloads — which may be good, considering Johnson’s injury history — it won’t be lack of touches that prevents Johnson from returning value on his fourth-round ADP. In four of Darrell Bevel’s last six seasons in Seattle, Detroit’s new offensive coordinator pushed the Seahawks to a top-three finish in rush attempts.

Of course, the performance of Johnson’s offensive line will be a big factor in him reaching his fantasy ceiling. Only six teams have fewer preseason rushes than Detroit but, for what it’s worth, the Lions have been top-10 in yards per carry and the grading of their linemen reflects that. I have the Lions’ run blocking as middle-of -the-pack with a healthy Frank Ragnow. If his recent ankle injury keeps him out for a long period of time Detroit has the depth to get by, but Ragnow is expected to solidify the center position. His second-year leap would be a big boost to Detroit’s ground game. All in all, I’m back on the bandwagon for Kerryon.

Here are four more RBs my outlook has evolved on over the course of the preseason …

Nick Chubb, Cleveland Browns

Is Nick Chubb going too early in the season’s final slate of fantasy drafts? According to Yahoo ADP, he’s risen to 10th overall after the trade of Duke Johnson to Houston. We’re rightfully optimistic about the Browns’ high-powered offense and Chubb’s prodigious talent. As a rookie, he scored 10 TDs and fell just short of a 1,000-yard season despite receiving just 16 carries in the Browns’ first six games. The stats still don’t show us how great this player can be.

However, alarm bells are clanging about Cleveland’s offensive line in both pass protection and run blocking. Only their intrastate rivals the Bengals have a lower preseason average than Cleveland’s 62 rushing yards per game. In the so-called “dress rehearsal” against Tampa Bay — a team that was very accommodating to opposing running games last year — the Browns’ line was pushed around. Joel Bitonio is nails but even J.C. Tretter, the line’s other solid starter, has struggled to open running lanes. Tretter is banged up and should return to form with better health, but that still leaves three-fifths of this line looking overwhelmed.

Whether by trade or savvy free-agent signings after final cuts, Cleveland’s front office needs to address this problem. I’ve gone from mildly concerned about the o-line to downright worried. Chubb’s ability is too otherworldly and the offense too loaded for him to be a total bust, but with terrible line play, we could see TD-dependent stat-lines from the former Georgia back.

Dalvin Cook, Minnesota Vikings

The fact that Minnesota has averaged over 40 rushing yards more than any other team this preseason should not be blown out of proportion — these aren’t real games, and the Vikings’ run blocking has not been great — but it’s certainly a statement of intent. This team wants to run. Dalvin Cook accounted for 85 yards on one scurry to the end zone, the kind of highlight-reel moment that injects helium into an ADP. Fantasy gamers have been rightly leery of Cook’s health history as a pro (it isn’t good), and the offensive line couldn’t block a telemarketer’s call last year. However, Cook has stayed off the injury report so far.

In July, I ranked Minnesota just outside the bottom-10 for expected run-blocking proficiency and while rookie center Garrett Bradbury has looked good in pass pro this exhibition season, his performance in the running game has been lackluster. Fortunately, for a runner as dynamic as Cook, the o-line doesn’t have to be a big plus as long as it’s not a big minus. “Respectable” should be fine, and the unit is trending that way.

The addition of advisor Gary Kubiak to the coaching staff should also pay dividends. In the 12 seasons he was an offensive coordinator, Kubiak’s teams never finished outside the top 10 in rushing yards, and they were top-five in nine of those years. Part of his influence on OC Kevin Stefanski should be a heavy dose of outside-zone, a blocking scheme Cook thrives in as a cutback runner. I’m willing to overlook my doubts about the o-line and trust the former FSU back to break off more big runs in 2019.

LeSean McCoy, Buffalo Bills

This could turn into an endorsement of Devin Singletary very quickly, but for now, it’s still Shady in the starting role. This offseason, Buffalo’s front office orchestrated the largest influx of offensive line talent in recent memory, bringing in a cavalcade of potential starters. The deep group has helped Bills running backs roll up the NFL’s fourth-most rushing yards this preseason. While a spate of injuries has kept the o-line from really getting in sync, the silver lining has been proving the versatility and resilience of the guys they’ve played. Competent results with all this shuffling are actually a good sign for the future.

With key additions Mitch Morse and Ty Nsekhe returning to practice, the arrow is pointing up for Buffalo’s line. They could be formidable sooner than I anticipated, which is good news for the backfield. If he isn’t traded or cut, McCoy should be the immediate beneficiary, but Singletary has turned heads this summer. He’s not a gifted athlete by any means, but with his vision and contact balance, the former Florida Atlantic Owl has demonstrated a remarkable ability to frustrate would-be tacklers. The rookie could seize the reins at any point.

Tony Pollard, Dallas Cowboys

The encouraging thing about the preseason for Dallas hasn’t been dominant performances from the running game; Cowboys backs have only mustered 87 rushing yards per game. What matters most is that their star-studded o-line hasn’t suffered any season-ending injuries. The starters do resemble temperamental sports cars that need to be saved for special occasions, not trips to the supermarket, but there is still hope the optimal lineup will be intact come Week 1. We weren’t sure that center Travis Frederick would pick up where he left off after missing the 2018 season due to illness, but he’s looked like the same studly player in his 38 preseason snaps. While Connor Williams continues to struggle in the running game, La’el Collins has looked dominant (he does have a groin injury to monitor).

A healthy Cowboys offensive line is bad news for run defenses. If and when Ezekiel Elliott returns, his outlook as a top pick is safe. Whether it’s Zeke or Tony Pollard toting the rock, there will be gaping holes for an RB to charge through. In the line’s last healthy year (2017) Dallas ranked fourth in Adjusted Line Yards, Football Outsiders’ run-blocking metric.

Jerry Jones has spun Pollard’s eye-opening preseason play to his advantage in contract-dispute posturing, but the Memphis rookie appears fully capable of exploiting the opportunities his blocking provides. If Zeke’s holdout extends into the regular season or he gets hurt, the Cowboys have found their most explosive understudy in years. I didn’t see that coming!

Listen to the Yahoo Fantasy Football Podcast