Physio Dave Williams told the club site: "That will probably be it for Alan for this season.
"The hamstring injury that bothered him a while back has reoccurred. It's tightened up and he will be out for a couple of weeks, which will take us past the end of the season.
"He came back from the initial hamstring problem quicker than he should have done because we needed him, we didn't have any other strikers fit.
"So the problem has never really gone away totally and it tightened up again at Watford.
"It's the hamstring problem that will stop him playing. He needs to go away and rest it. His nose is broken as well, but that wouldn't keep him out."