Claudio Ranieri believes defeat by Chelsea could deal a fatal blow to Manchester United's title ambitions.
Ranieri says Sunday's game at Old Trafford is crucial to United's Premiership hopes, even though they are three points clear at the top.
"If Manchester United lose, they might also lose conviction and belief," the former Chelsea boss told BBC Sport.
"Chelsea can lose and they will still fight to the end of the season. They expect to win the Premiership again."
Ranieri, who managed Chelsea from 2000 to 2004, added: "United don't have that same belief after missing out for the last few seasons.
"So this is a very important match for them. If they win, it will give them even more ambition. If they lose it could deflate them."
"Shevchenko is a fantastic player and will score a lot of goals by the end of the season"
The 55-year-old believes Chelsea are finding their best form after a difficult start to the season.
"At the start of the season, the change from 4-3-3 to a diamond formation was a little difficult for the players," he said.
"When you change the system, it's very difficult to do everything well.
"Michael Ballack and Andriy Shevchenko also arrived from foreign leagues. Sometimes when a player changes countries he needs six or seven months to adjust.
"Shevchenko is a fantastic player and will score a lot of goals by the end of the season.
"And Ballack is slowly, slowly getting better. Now there is good movement between him and Lampard in midfield. Now Chelsea are playing well and they're winning."
But another player has impressed Ranieri most of all.
"Drogba has been fantastic. It also took him time to adapt to the Premiership, but now he looks like he was designed to play in England.
"He is so strong, fast and powerful."
Ranieri thinks the title race is already a straight fight between Chelsea and United.
"I think it will be a fight between them this season," he said.
"I'm not surprised that United have come back so strongly this season.
"They had a very good start, which was so important. Scholes has come back and is a fantastic player.
"Solskjaer also returned after a long time out and I think that lifted the whole squad. And the young players like Ronaldo have also got more experience."