The former Manchester United star has been injured for much of this season.
But the 30-year-old is in line to come back against Brighton in the Premier League on Saturday.
It is a must win game for Watford.
"We will assess Sarr tomorrow and make a decision on that.
"It is always a danger to put players at risk with soft-tissue injuries. With sprinters, and Ismaila is certainly a sprinter, there is more danger.
"With any injury like that you have to calculate the risk and decide whether the risk is worth taking.
"We have got 13 games left so it is prudent to make sure that when players like that are back, they stay back."