Shinji Kagawa will miss Manchester United's top-of-the-table game with Chelsea on Sunday after the Japanese midfielder was ruled out for up to a month with a knee injury.
United manager Alex Ferguson revealed on Friday he should have substituted Kagawa sooner after the player suffered the problem in the first half during the 3-2 Champions League win over Braga on Tuesday.
Kagawa was eventually replaced at half-time by Nani.
"Kagawa is going to be out for three to four weeks," Ferguson told a press conference in Manchester. "I think playing on didn't help the situation although he did well to play on with it to be honest."
United start the weekend four points behind leaders Chelsea after eight games, but Ferguson's side have won 10 of the last 11 games in all competitions.
"It is always a very difficult game for both Chelsea and ourselves, it's never easy," added Ferguson.
"Chelsea are top of the league and have had a great start to the season. They are playing an interesting formation and it's something we've got to deal with."
It is the first of three games in a week for United which could shape their season. They return to Stamford Bridge on Wednesday in the fourth round of the League Cup before entertaining Arsenal in the league on November 3.
United have managed just three clean sheets in 12 games in all competitions this season, but are set to welcome back Rio Ferdinand and Patrice Evra after both were rested as Ferguson's side came from 2-0 behind to beat Portuguese side Braga 3-2 in the Champions League on Tuesday.
It will be Ferdinand's first game since Ferguson accused the defender of embarrassing him after Ferdinand refused to wear a 'Kick It Out' anti-racism T-shirt in the warm-up prior to the 4-2 home win over Stoke City last Saturday.
Ferguson said on Monday his relationship with Ferdinand was back to normal after clear-the-air talks.
"I think it's bubbling along that race issue," added Ferguson on Friday. "It's not completely gone away. But the thing is the awareness is getting stronger and the Professional Footballers Association are, I'm sure, doing their best to support it.
"The FA have to do more with UEFA and FIFA. That's without question."
Ferguson has to decide whether to hand Javier Hernandez his first league start since September 15 after the Mexican scored twice against Braga at Old Trafford.
Hernandez, 24, netted United's equaliser as Ferguson's side came from 3-0 behind to draw 3-3 on their last visit to Chelsea in February.
However, Ferguson is likely to name attackers Wayne Rooney, who turned 27 on Wednesday, and Robin Van Persie, who has scored six times in eight league appearances since moving from Arsenal in August.
Ferguson said: "The good thing about Javier is that he is improving. I think the bonus for him is that he got a rest in the summer. It wasn't exhausting as it had been for the previous three summers for him. That's helping him."
United are unbeaten in their last five meetings with Chelsea, winning four of them.
Goalkeeper Anders Lindegaard is out with a thumb injury while long-term absentees Nemanja Vidic (knee), Phil Jones (knee) and Chris Smalling (metatarsal fracture) remain sidelined.