Manchester United goalkeeper Ben Foster admits he's unsure when he will be back after knee surgery.

The Reds' stopper, who has spent the last two seasons on loan at Watford, suffered pain in his right knee towards the end of last season.

He will go under the knife in the next couple of weeks in a bid to cure the complaint which is likely to sideline the 24-year-old for the opening part of the new season.

"It's irritating rather than really painful. I thought it had cleared up after it troubled me last season," Foster told the Daily Mirror.

"I'm having it scanned and we'll take it from there but it's very early to say what the outcome may be.

"I suppose if something has to be done this is the best time because I've got loads of time before the start of the new season."