Mateo Kovacic has challenged Chelsea to "wake up" and take charge of their battle for Champions League qualification.
The Blues could see Tottenham leapfrog them into the top four with a loss to their bitter rivals at Stamford Bridge on Saturday.
Chelsea held a six-point lead over Jose Mourinho's men after winning 2-0 in north London on December 22, but that gap has evaporated amid the Blues' patchy form.
Frank Lampard's side suffered a ninth league loss of the campaign with Monday's fractious 2-0 home defeat by Manchester United, leaving midfielder Kovacic imploring the Stamford Bridge men to snap out of their malaise.
"We need to improve a lot because we have tough games ahead of us and we have to be focused," said Kovacic.
"We have to train hard every day, to prepare our games better and try to avoid our mistakes because we are making too many mistakes in these kind of matches and you get punished.
"We have a good week ahead of us to prepare for Tottenham and to play a good match.
"We know this match means a lot for us because Tottenham are close behind us so we need to prepare ourselves quite a lot to win this game at home.
"We are still fourth so we are still in a good position, but we need to wake up because they are coming from behind.
"We need to play these next games like we were playing in the beginning of the season and change some things as well, obviously."
Chelsea endured a third loss to United this season, with the Red Devils claiming their first league double over the Blues in 32 years.
Anthony Martial and Harry Maguire scored for the visitors, while Chelsea had goals from Kurt Zouma and Olivier Giroud chalked off by the video assistant referee (VAR).
Maguire arguably should not have been on the pitch to head home his goal, having somehow escaped a red card despite kicking out at Michy Batshuayi.
Croatia midfielder Kovacic lamented yet another error-strewn Chelsea performance, admitting there is no more time to lose for the Blues to sharpen up.
"We played well in the beginning but it's always the same against United; we play good, we create chances, we don't score and then they score from one opportunity," Kovacic told Chelsea's website.
"We don't have the three points and that's the most important thing. It was a good game from us, we played well, like we have for more or less the whole season, but we made some silly mistakes and we lost the points.
"We are still a very young team and we need to improve a lot. Now we will see if we are a great team because we have some tough matches to fight for the fourth position and we need to show ourselves."
Ole Gunnar Solskjaer was left delighted with a battling win on the road, but quickly turned his attention to scheduling frustrations.
United will travel to Belgium to take on Club Brugge in the Europa League on Thursday night, with boss Solskjaer baffled to have had to play in the league just three days earlier.
"We have to work hard to recover now because we've got another game on Thursday," said Solskjaer.
"So thanks for the help, for all the recovery we get in Europe.
"I saw Jose Mourinho say something about that as well. We don't really get too much help to prepare for Europe.
"How we didn't play this game against Chelsea on Friday, Saturday, or at least Sunday, is beyond me.
"I don't like to complain, you know that. So we've got to get on with it."
