Premier League titles are not won and lost in October. And on an October Saturday when the title race will be a major talking point, that’s a very important caveat to keep in mind.

But Premier League titles are not won and lost solely in May either. Nor in April, nor in March. They are won and lost over 38 weeks, each having the ability to hold equal importance. Every weekend can matter just the same. And this one – specifically, two hours on the afternoon of Saturday, Oct. 21 – seems particularly meaningful.

Because it brought the first major twist in a tale whose opening chapter was written before the season even began. While Manchester City kept up its exhilarating pace in what was shaping up to be a two-team title race, cross-town rival Manchester United came to a screeching halt. Its 2-1 loss at Huddersfield was the first league defeat for either of the Manchester clubs and opened up a five-point gap between them. It also exposed a critical difference between the two that could keep the table looking this way come May.

City’s fixture, at home again Burnley, was the easier of the two. But both were very winnable. Both Jose Mourinho and Pep Guardiola, therefore, opted for some squad rotation. Mourinho left out Henrikh Mkhitaryan and Marcus Rashford. Guardiola rested Gabriel Jesus and Raheem Sterling.

The two decisions, though, were and are far from equivalents. Guardiola brought in Sergio Aguero and Bernardo Silva, and City didn’t miss a beat. It had over 75 percent possession, and 10 shots on target to Burnley’s zero in a 3-0 victory.

Mourinho, on the other hand, turned to an attacking midfield line of Juan Mata, Jesse Lingard and Anthony Martial. United was both mistake-prone and flat-out poor in the first half. Mata was culpable on the first Huddersfield goal for a giveaway in midfield. Aaron Mooy nicked the ball off him, drove forward, and eventually finished the move to give the hosts a lead:

United was also weakened at the back when Phil Jones was forced off by an injury. Victor Lindelof replaced him, and five minutes after Mata’s mistake, the young Swede inexplicably ducked a header. Huddersfield keeper Jonas Lossl’s punt led directly to the Terriers’ second:

An equally telling storyline, though, was United’s attack. It was mostly dormant. The Red Devils bossed possession (78 percent), especially in the second half. But they didn’t manufacture many, if any, chances of note. They totaled three shots on target, equal to Huddersfield’s tally, over the 90 minutes.

Mourinho decided he had seen enough at halftime. He brought on his two regulars, Mkhitaryan and Rashford, for Mata and Martial. Rashord halved the deficit with a 78th-minute header. But United didn’t deserve anything from the game, and it didn’t get anything.

Huddersfield, on the other hand, deserves plenty of credit. Mourinho was quick to give it to them. “The best team won,” he said after the match. “Simple.” It was the club’s first win over United since 1952. The players’ and fans’ rain-drenched celebrations were well-earned.

But their visitors were equally responsible for the 2-1 score line. “One team was to compete, and one team was to be aggressive, and one team was to give absolutely everything,” Mourinho said. “Another team was not ready for that.”

City, meanwhile, only led 1-0 after an hour. But unlike United, the Citizens were dominant. The two Expected Goals maps are telling:

Bernardo Silva won a penalty late in the first half to give them the lead. Nicolas Otamendi put the game away in the 73rd minute, and a devastating break minutes later allowed Leroy Sane to make it three.

United, after facing the kindest of schedules to open the season, now finds itself five points back in the Premier League. It is by no means an insurmountable lead. And injuries – especially one to Paul Pogba – have been a key factor in United’s last two Premier League results.

But Saturday told us about margins. United’s margin for error, if it wants to perform at a title-winning level over 38 games, is significantly slimmer than City’s. Add in the margin that now separates the two teams in the standings, and Saturday’s results were very significant.

Other Premier League scores

Chelsea 4-2 Watford
Newcastle 1-0 Crystal Palace
Stoke City 1-2 Bournemouth
Swansea City 1-2 Leicester City

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Henry Bushnell covers soccer – the U.S. national teams, the Premier League, and much, much more – for FC Yahoo and Yahoo Sports. Have a tip? Question? Comment? Email him at henrydbushnell@gmail.com or follow him on Twitter @HenryBushnell.