Manchester United manager Sir Alex Ferguson has revealed that midfielder Shinji Kagawa is likely to be unavailable for a month due to a knee injury.

The Japan international was a doubt for this weekend's Premier League battle against Chelsea after picking up the knee problem in United's 3-2 win over Braga in the Champions League on Tuesday, but it has been confirmed that the former Borussia Dortmund star will be sidelined for a longer period of time.

"It's a knee injury similar to the one Ashley Young had but not as bad," Ferguson told the club's official website. "I think playing on with the injury did not help the situation for him, even though he did well to play on with it. It's disappointing for him."

Meanwhile, Ferguson has confirmed that defender Chris Smalling is back in full training following a stint on the injury list.