Sir Alex Ferguson believes that Chelsea owner Roman Abramovich will demand even more from manager Roberto Di Matteo next season.
The Italian took over interim charge of the Blues in March and went on to guide the club to success in both the FA Cup and Champions League.
As a result, Abramovich opted to give Di Matteo the job on a permanent basis earlier this summer.
However, Manchester United boss Ferguson has warned Di Matteo that he is now under pressure to deliver again.
"I think that probably the owner has his enthusiasm back, after winning the Champions League," Ferguson told reporters. "I think there will be more expectation from him now.
"It's the trophy they always wanted to win and he's invested in the team again.
"It's a challenge for us. We will always respect Chelsea, but you have to wait and see with these players in terms of how good they are."
Ferguson's United are due to travel to Stamford Bridge on Sunday, October 10.
