Manchester United vs. Arsenal: 5 Things We Learned at Old Trafford

We knew Arsenal were under-strength for their trip to Old Trafford, but nobody predicted they'd go down in quite such spectacular fashion to a rampant and ruthless Manchester United.

In an afternoon to rank alongside the worst in the club's history, Arsene Wenger's men were humiliated 8-2. They were lucky to get two and United could have scored 15.

It was the first time the Gunners have conceded eight since the late 19th century and the worst defeat of manager Arsene Wenger's career.

Here are five things we learned.

1. Arsenal Are Unravelling

Arsenal haven't had the best start to the new campaign. Cesc Fabregas and Samir Nasri have been sold, their squad is creaking under an injury crisis and they've taken just a solitary point from their first three Premier League games.

However, what truly exacerbated their problems yesterday was a complete lack of discipline. Players were making reckless tackles, defenders were making basic errors of concentration and at times they looked like a team of pouting teenagers.

Arsenal have now had three men sent off in their first three league games. As onlookers were quick to point out yesterday, Arsene Wenger's team better hope they're not relying on the Fair Play League for a place in Europe next season.

2. David De Gea Picked the Right Club

When you win 8-2 there's unlikely to be much attention on your goalkeeper.

David De Gea saved a penalty yesterday, but he also let in another soft goal when Theo Walcott's angled shot went through his legs.

De Gea can count himself lucky none of his mistakes have thus far cost United a victory. Sir Alex Ferguson has given him the luxury of a bedding-in period with a tolerance for howlers.

United fans will be hoping De Gea is getting all of his errors out of his system before the club face a true test.

3. Wayne Rooney Can Win It All This Season

Wayne Rooney made a slow start last season, coming off a poor performance at the 2010 World Cup and dogged by scandal in his private life.

However, 12 months on and we're seeing why United fans are calling him "the white Pele" again. Rooney has scored five times in three Premier League games and looks like a man on a mission.

His creativity, finishing and work ethic were unmatched yesterday. On his day he's the best in the Premier League.

If he keeps up this kind of momentum, he'll score 35 this season and help United win every trophy in sight.

4. Arsenal Need a Defender

Arsene Wenger said Arsenal were closing in on Monaco's South Korean striker Park Chu-Young last night. That's all well and good, but there's no doubting where they most need reinforcement.

Judging on their performance on Sunday, Wenger's defense desperately needs an experienced head (or four) to ensure a defeat like that never happens again on his watch.

Arsenal once boasted the meanest, more organized back line in England. At Old Trafford, their back four looked like they belonged in a pub team.

5. The Title Will Be Won in Manchester

From what we've seen thus far, the Premier League is looking like a straight shoot-out between Manchester United and Manchester City.

United and City combined for an aggregate 13-3 win against Arsenal and Tottenham yesterday, further stamping their authority on the also-rans they look capable of leaving behind this season.

Meanwhile, Chelsea's 3-1 win against Norwich was less than convincing. Liverpool were impressive against Bolton, but it's hard to see them matching the likes of City and United over an entire campaign.