The forward spent five months on the sidelines with an Achilles injury, but has managed just two goals in 13 appearances since returning to the side.
"He's not 100%," PA Sport quotes Allardyce as saying. "He's had no pre-season at all, he's worked very hard, but getting into games at this stage of the season after so long out is hard.
"It's hard to get that ultimate sharpness that you need. Hopefully he can find it."
The Hammers suffered their third defeat in a row on Saturday in a 1-0 loss to Crystal Palace.