Sir Alex Ferguson admits Jose Mourinho's dramatic departure from Chelsea this week came as a major disappointment to the Manchester United manager.
Mourinho was due to take his side to Old Trafford on Sunday to face Ferguson's United before the Portugese unexpectedly left his post on Thursday.
Ferguson attempted to play down the impact of Mourinho's absence on the weekend clash between the two teams expected to challenge most strongly for this season's Premiership title.
But while the United manager insisted the make-up of the Chelsea side will be unaffected by the changes in the dug-out, he admitted Mourinho's replacement, Avram Grant, faces a daunting task attempting to emulate the success of his predecessor.
And Ferguson - who acknowledged he will miss the chance to extend his three-year rivalry with Mourinho - believes the English game will be poorer for the loss of the controversial former FC Porto coach.
"Jose leaving is a disappointment for the game," Ferguson said. "He was terrific for football and terrific for Chelsea and I enjoyed the competition with him.
"He was something fresh and new for the game. But I wish him well and think he has been good for the game.
"He has been fantastic, unparalleled success and it's a test for whoever replaces him. At the moment it's Avram Grant and it's a big test for him.
"But we are looking at ourselves and not at Chelsea and what it does to them doesn't matter to me. We are quite focused on our own game because we are still playing Chelsea and it's still the same group of players and that in itself is a challenge for us.
"It's always a difficult match and if you look at the record of the two teams over the last few years, there's nothing in it. Very few goals are scored and it's always a tight, tight game."
Both teams have made an indifferent start to the campaign compared to recent seasons and while victory would inflict a major blow on the clubs' hopes of quickly regaining ground, Ferguson attempted to play down the significance of the fixture.
"When you play Chelsea it's always a going to be a big game," he added. "I don't think it's going to be a defining moment in the league but nevertheless it's still going to be important but at the end of the year you may say it was an important result.
"The recent results have given us a platform for Sunday. Both Chelsea and ourselves are in the same position, we have both dropped points.
"We dropped our points early on and now we have had four wins in a row and if we get to that consistency we'll get back to our normal position up towards the top of the league."
United have no new injuries following the midweek victory over Sporting Lisbon and the manager must decide on his striking options after the return to fitness of Wayne Rooney and Louis Saha.
Ferguson admitted that with Carlos Tevez and Anderson still battling to reach their peak, he is undecided about which pairing represents his most potent strike-force.
"I don't know what the best attacking option will be because I don't have any evidence at the moment and what we will do on Sunday may be different to what we will do in the following game," the manager said.
"If Saha stays fit, Tevez gets his fitness and Rooney is going to get fitter it gives me tremendous options. We are on the road to finding these things out.
"What it does is give me a far better squad and more ammunition to work with and that's really important because we maybe didn't have it last year."