The San Francisco 49ers' offense was stuck in a little bit of a rut Sunday night against the Dallas Cowboys, but an unlikely hero helped them turn it around.

In the first half of a 30-24 win over the Cowboys, the 49ers scored only six points. Their run game in particular struggled with running back Jordan Mason churning out only 18 yards on six carries in the first quarter and change.

A shoulder injury wound up forcing Mason into the locker room before halftime, opening the door for rookie fourth-round pick Isaac Guerendo. He entered the game with just 23 carries on the season, including only one in Week 7 against the Chiefs.

Sunday wasn't the first time Guerendo got an opportunity for extended playing time, but it was certainly his best outing. He rushed for 85 yards on 14 carries and scored his first-career touchdown. He also got to 85 yards with a long run of just 19.

Head coach Kyle Shanahan told reporters after the game that the improvement for Guerendo has been clear.

“I think he's learned from each run he’s gotten," Shanahan said. "There are a couple runs earlier in the year that he did alright on, but we grade him pretty hard, everybody pretty hard. And he had a couple, like four-yard runs we thought he could have got seven yards on. We go pretty hard on him. And then you watch how he takes that to practice and everything we've really kind of been pushing him with, he's been real coachable, he's not a sensitive guy, he's real detailed and he's very poised for a rookie. Things aren't too big for him and didn't get a lot of preseason, but once he got healthy there and throughout this year, I feel he’s gotten better each week.”

Guerendo also played a big role in the 49ers' Week 6 win over the Seattle Seahawks, but he struggled to find running room in that game before a 76-yard run late in the fourth quarter helped the 49ers seal the victory. He finished the game with 10 carries for 99 yards.

He was more consistent Sunday night with runs of 19 and 15 yards on his first two touches. He flashed his speed and his power, and in general looked more patient in waiting for running lanes to develop.

Shanahan also said Guerendo was running well enough that they didn't even try to put Mason back in the game after he came out for the second half with his helmet on -- a sign he was ready to go.

Guerendo rewarded the 49ers' faith in him by going for 47 yards and a touchdown on 11 carries in the second half. He also contributed as a pass catcher, hauling in three of his four targets for 17 yards.

The talent has always been apparent with Guerendo, he just wasn't catching on the way we've seen other RBs do in Shanahan's scheme. Sunday night was his coming out party and it came in a game where the 49ers desperately needed their run game to get rolling to help them turn things around after a rough first half.

With Christian McCaffrey set to return soon, Guerendo may not get another opportunity as a lead RB this season, but he took his opportunity Sunday night and played well enough to be a major cog in a huge 49ers victory.

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This article originally appeared on Niners Wire: 49ers offense gets boost from unlikely source vs. Cowboys