Watford physio Andy Rolls on Watford's website:
"Ben Foster has been worked exceptionally hard with his knee over the last fortnight.
"He's had a niggle there for the last few weeks and we've been very aware of that.
"Over this period, we've been in constant communication with Manchester United and we felt that by resting from the Middlesbrough game it would sort him out.
"Unfortunately, during the game at Portsmouth on Saturday, his knee semi-locked on him while he was taking a goal kick and he wasn't happy with how it felt at all.
"In liaison with Manchester United, he had a minor operation on Monday and has already begun his rehabilitation."