The striker had to come off at half-time after taking a couple of boneshaking tackles in the first half of our 1-0 Premiership defeat.
Teddy Sheringham (groin), Dean Richards (hamstring) Ben Thatcher and Oyvind Leonhardsen (both calf) are also fighting to be fit.
Glenn revealed: "Les played with a slight Achilles. He took a blow to it which has made it worse and he's another one who is going to be doubtful for next week.
"It's one after another at the moment and a big problem for us because we can't really afford to pick up any more. We had 16 senior players available for this game. That's all we've got left.
"It's a big test of character. There were a lot of stern words in that dressing room but we have to fight back."
Les Ferdinand is struggling for Sunday's clash at Fulham after aggravating an Achilles problem against Charlton last night.