James has been ruled out with a hamstring injury while Mount has been sidelined with an abdominal problem, although he has an outside chance of returning for the last game of the season.
"James will be unavailable for the rest of the season. Mount the same, he might make the last match but that's unlikely. That's obviously disappointing for us," Lampard told reporters ahead of Wednesday's Premier League clash against Brentford.
"Kai (Havertz) is still struggling with his knee but should be available for the end of the week."
Chelsea, who are 11th in the league standings and at risk of their worst domestic league finish in nearly 30 years, will look to end their seven-game winless run in all competitions against Brentford.
Chelsea are five points behind their west London rivals, who have played a game more, as Lampard seeks for his first win since returning to Stamford Bridge following Graham Potter's departure.
"I wouldn't expect anything in football, but we have come into a club with big difficulties with not a lot of time," said Lampard, who lost his first four games.
"A lot of factors as to the four games I have been here, the team and club have been struggling for a while, we are here to help. Things won't change in a short time."
Chelsea have nothing left to play for this season with qualifying for Europe a tough task but Lampard said motivating players would not be a problem.
"I don't have a mentality that there is nothing to play for. They have been fortunate at Chelsea to have had that for 15, 20 years, but it's a reality when you have a long career, you won't compete every season," he said.
"We now have seven games, three against the top four, so will be interesting to see how the players react."
