The European champions have only dropped two Barclays Premier League points all season and sit proudly at the summit, four points clear of the two Manchester sides.
With Eden Hazard, Juan Mata and Oscar all impressing with their clever play, it promises to be a real test for the Reds.
"It’s always a very difficult game at Chelsea for both sides," explained Sir Alex. "You’ve got two good teams. They’re top of the league after a great start to the season – they’ve done really well. They've played an interesting formation and it’s something we have to deal with.
"They've brought Oscar and Hazard into the team – they’re similar types of players, I think. They're adapting to having players like Mata also in the same position - there is a situation where they're in behind the front player all the time. It’s not an easy system to deal with, but we just have to find a way.
"They’re all similar players, they play in behind the main striker. Of course they don't have Didier Drogba so their game had to change anyway. They always had that alternative route of playing with Drogba up front – they could play it long or they could play it through midfield. Now it’s straightforward, it’s through midfield all the time."