The injury occurred 35 minutes into Bent's first start since October, having been replaced as the side's striking spearhead by Belgian international Christian Benteke.
"He's had a scan and we've got to wait to see how it settles down," Lambert told a news conference.
The injury means the former England player is likely to miss the busy Christmas period.
Captain Ron Vlaar will also miss Saturday's Premier League trip to Liverpool with a calf strain.
The Dutch defender has not played since his side's goalless draw against Arsenal on Nov. 24.
Villa are one place above the relegation zone in 17th place but have only lost twice in the last eight matches, to Manchester United and Manchester City.