Mount's Champions League debut lasted nine minutes after a horror tackle by Valencia's Francis Coquelin in Chelsea's 1-0 defeat at Stamford Bridge on Tuesday.
"We have to assess it over the next 24-48 hours to see the scale of the injury," Lampard told a news conference.
"We don't know how bad it is. It is an ankle injury and it is a shame. He's been playing so well and he started well tonight."
Following an impressive loan spell at Derby County last season, Mount made his Chelsea first team debut at the start of the 2019-20 campaign and went on to score three goals from five league starts.
Chelsea, who are currently sixth in the table with eight points from five matches, will look to dent league leaders Liverpool's perfect start to the domestic campaign.