The England international is expected to return in two weeks.
Watford physio Andy Rolls told the club’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 – a slight irritation to the meniscus in his knee.
“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.”
