Carlo Ancelotti has admitted his Chelsea side must win 13 of their remaining 16 games if they want to defend their Premier League title.
Manchester United stretched their club-record unbeaten start to the season to 22 games on Saturday to move 10 points clear of Chelsea, but Ancelotti is convinced they will not emulate Arsenal's 2004 'Invincibles'.
"I think it's impossible because if we want to come back to fight for the title we have to beat Man United," Ancelotti said.
As well as beating United home and away, Ancelotti admits Chelsea must also win their head-to-heads with third-placed Manchester City and fifth-placed Tottenham at Stamford Bridge in the second half of the season if they are to defend their crown.
"We have to win these games. That is the key to come back to win the title," said Ancelotti, who admitted he had not been completely satisfied with his team's performance since they beat second-placed Arsenal almost four months ago.
"We have to play twice against Man Utd, we have to play Man City, Tottenham, Liverpool."
Ancelotti has already admitted Chelsea must win at least 13 of their remaining 16 games to retain the title and knows there is almost no margin for error, as he found out in similar circumstances while captain of Roma 25 years ago.
"In 1986 we needed to close a gap of eight points when it was two points for a win," he said of his side's battle to overhaul Juventus.
"We ran for two months closing the points, eight, seven, six, five.
"The last but one game we were level and we lost 3-2 at home to the bottom team, Lecce.
"Before, there was a celebration in the stadium. The coach of Lecce even said, 'I am a fan of Roma. I hate Juventus'.
"We were not able to win and Juventus won the title. This is life."
Ancelotti accepts the Blues are currently no match for unbeaten Premier League leaders Manchester United when it comes to grinding out results.
The Italian's claim casts huge doubt on the champions' hopes of staging an extraordinary comeback in the title race, with the team that finishes top often proving the best at picking up points when playing badly.
However, Ancelotti has no intention of changing the approach which brought Chelsea the double last season.
"For Chelsea to win a game, we have to play well, play good football," said Ancelotti, whose side travel to Bolton on Monday night 10 points adrift of United.
"We are not able to win a game if we are not able to play well. We played very well last season and when we didn't play well, we lost the game the same way we have lost them this season.
"Our aim is to improve our football. This is the way to win the game. It is our philosophy.
"We don't want to stay back and maybe use the long ball or use the counter-attack.
"Maybe this is also the reason that if we don't play well, we are vulnerable."
