Sir Alex admits United are facing a difficult test against Arsenal at the Emirates Stadium on Sunday.
The Reds hammered Arsene Wenger's side 8-2 at Old Trafford in August and the Gunners will be determined to avenge that heavy beating. The manager believes it will understandably be a much tighter and tougher affair in the capital.
"Sunday's game is the start of a hectic spell for us," he declared. "I hope we come through the programme well because it could decide a lot of the season.
"It's always a hard game with Arsenal and it should be a good one. They recovered well from the 8-2, had a really good spell and qualified for the next stage of the Champions League.
"They've had to contend with some injuries to defenders, particularly their two regular full-backs - [Bacary] Sagna and [Kieran] Gibbs have both been injured for quite a while now and they've had to contend with that which isn't easy. I think they've equipped themselves well.
"Revenge? It has never been part of my career to think about it. It just doesn't apply itself to me but maybe others do. Everyone's different in that way.
"I certainly don't think the game will be as open as Old Trafford earlier in the season. It will be a difficult game. They've only lost six goals at home this season so we're well aware it will be a hard game."