Mount has been a revelation for the Blues since breaking into the first-team, but his Champions League debut did not get off to a good start after former Arsenal midfielder Frances Coquelin left his studs on the inside of the youngster's shin.
Lampard does not yet know the extent of the injury and is keeping his fingers crossed for good news.
"It was a shame, he's been playing well and he started this game well," Lampard said after the 1-0 defeat against the La Liga side.
It capped a miserable night for Chelsea as Valencia left Stamford Bridge with all three points thanks to a goal from winger Rodrigo.
Chelsea had a chance to equalise from the penalty spot, but Ross Barkley missed following an argument with Jorginho and Willian over who should take it.
Barkley, on as a substitute, assumed responsibility for the penalty but only after some heated debate with his team-mates.
The miss condemned Lampard to defeat on his Champions League debut as Chelsea boss, but speaking after the game he confirmed Barkley was Chelsea's designated penalty-taker.
"Ross is the penalty taker. He took some in pre-season and he has been when he has started, and he was today when he came on," he said.
"I don't know what conversation was, but Jorginho and Willian were penalty-takers until Ross came on the pitch."
Lampard, who played 102 Champions League matches for Chelsea and captained them to their 2012 triumph, admitted his young team were given a harsh lesson on life among Europe's elite.
He fielded four Champions League debutants, and Willian and Tammy Abraham went close to breaking the deadlock numerous times before Rodrigo pounced from a set-piece with 16 minutes to go.
"We probably should have won, or at least got a draw," added Lampard. "We created enough chances, we had the penalty, and we conceded with their one shot on goal."