The 28-year-old was left out of Villa's 1-0 win over Sunderland because of an injury blow he picked up during training on Friday.
"It is the same ankle he hurt against Wigan [Athletic in February]," The Independent quotes Lambert as saying. "I know it was serious then.
"Is it serious? I don't know yet. He will get treatment on it during the week and we will see. He was really hobbling on Friday and we couldn't take a chance on him with Sunderland."
Bent was out of action for three months last season when he suffered a similar injury to the same ankle.