The striker has a groin problem that is giving him a referred pain in the stomach area after playing for 45 minutes at Old Trafford on Wednesday.
Darren Anderton is in a similar position as his hamstring strain improves and Tim Sherwood is the other doubt. Steffen Iversen (groin) is definitely out.
Glenn said: "We've still got slight problems with Darren, Tim Sherwood and Les, who has a stomach and groin problem. It's a referred pain but he's improving.
"He played half at Old Trafford, aggravated it a little, but we're hoping he'll be okay. We've got an extra day with the game on Sunday and that will come in handy.
"With Steffen Iversen out we'll give him as much time as we can."
Chelsea's Eidur Gudjohnsen should be okay for the tie after he took a blow to the head in the win over Fulham.
Mario Stanic is available after suspension for Claudio Ranieri's team but there is a doubt about Emmanuel Petit after he limped out of the action on Wednesday.
Boudewijn Zenden (knee), John Terry (broken bone in foot) and Albert Ferrer (calf) are all out.
Glenn Hoddle will give Les Ferdinand as long as possible to declare himself fit for Sunday's FA Cup clash against Chelsea.