Manchester City preserved their 100 percent record in the Premier League this weekend, Chelsea lost theirs and champions Leicester City fell to a second defeat in four matches.
Here are five things we learnt:
Manchester United are a work in progress
After winning their first three league games, Manchester United were installed as slight favourites ahead of the derby against Manchester City. But they were badly exposed in the first half by Pep Guardiola's fleet-footed side, with several players looking off the pace. The ponderous Henrikh Mkhitaryan was taken off at half-time, along with Jesse Lingard, while Paul Pogba failed to assert himself and manager Jose Mourinho had stinging words of criticism for centre-backs Eric Bailly and Daley Blind. Once again, it took the introduction of fearless 18-year-old Marcus Rashford to enliven United. Mourinho, whose side lost 2-1, may need to find a permanent starting role for Rashford if United are to avoid losing touch with City.
Bravo could be Manchester City's weak link
Manchester City were 2-0 up and cruising at Old Trafford until their new goalkeeper Claudio Bravo dropped a high cross from Wayne Rooney, allowing Zlatan Ibrahimovic to halve the deficit. The Chile international was signed in large part due to his quality on the ball, but he showed serious deficiencies when it came to handling crosses. His ease on the ball almost came back to bite him, meanwhile, when he overran the ball inside the penalty area and launched into a risky challenge on Rooney. Fortunately for him, and manager Guardiola, referee Mark Clattenburg did not award a penalty. Guardiola praised Bravo for showing "personality" after his first-half error, but opposition sides will be rubbing their hands at the thought of the former Barcelona goalkeeper's vulnerability in the air.
New stand, new hope for Liverpool
Jumping for joy on the touchline, Jurgen Klopp's manic celebrations were replicated all around him as Liverpool's biggest crowd since 1977 hailed a 4-1 thrashing of champions Leicester. The last time an attendance of 53,000 or more packed into Anfield, Liverpool were English champions under the leadership of the softly spoken Bob Paisley. Now, thanks to a significant redevelopment of the historic stadium's Main Stand and the shrewd leadership of the far more voluble Klopp, Liverpool can once again dream of emulating the feats of Kenny Dalglish, Phil Thompson and company. Judging by the way the Reds tore into Leicester, with goals from Roberto Firmino, Sadio Mane and Adam Lallana, Klopp's side could yet mount a serious title challenge this season.
Arsenal have problems up front
Arsenal took three points against Southampton, but they needed a stoppage-time Santi Cazorla penalty to prevail and there was no disguising their deficiencies in attack. Debutant striker Lucas Perez was largely anonymous, with Arsenal manager Arsene Wenger saying he had found the pace compared to La Liga a surprise. Olivier Giroud, a second-half substitute at the Emirates Stadium, is yet to score this season and Alexis Sanchez has found the net only once in four outings. Southampton remain without a win under manager Claude Puel, but if Shane Long had made more of his second-half chances, they might have caused an upset.
Chelsea are still fragile
After last season's dismal 10th-place finish, Chelsea made a storming start to the campaign under new manager Antonio Conte, winning their first three games. They were 1-0 up at Swansea City and in total control of the match when the vulnerability that plagued them throughout last season resurfaced. Gylfi Sigurdsson equalised with a penalty after being brought down by a needless challenge from Chelsea goalkeeper Thibaut Courtois before Leroy Fer robbed Gary Cahill -- fouling him in the process, as television replays would reveal -- to put Swansea ahead. Diego Costa's acrobatic second goal, in the 81st minute, rescued a point, but Chelsea's jittery second-half display suggested last season's disappointments have not yet been fully digested.