The Blues boss is considering giving his new goalkeeper a debut in the London derby on Tuesday night after signing him last week
The 28-year-old made the £22 million (€24m/$28m) move from Rennes on Thursday last week, and he could be thrown straight into the Blues starting line up against Jose Mourinho’s side.
Hakim Ziyech and Christian Pulisic remain out injured with knee and hamstring problems respectively, but all other Chelsea players are fit including Ben Chilwell, who could make his first start since joining in a £50m ($65m) move from Leicester City.
Lampard confirmed the news about the status of his team ahead of the fourth-round match at Tottenham Hotspur Stadium.
“Mendy and Chilwell are in the squad tomorrow,” Lampard confirmed. “They are fit and we will see if they start or not. Ziyech and Pulisic are not far away but they are still in a recovery phase. It is too early for them both.”
Mendy’s arrival is timely with Kepa Arrizabalaga struggling for form and Willy Caballero having conceded three goals from three shots on target in the 3-3 draw with West Bromwich Albion on Saturday.
Lampard said Mendy has fitted in well with the group ahead of the match and he has seen his character have a good influence on the maligned group of goalkeepers at Cobham.
“It is all very positive from the early signs and interactions,” he added. “Just as a man on the training pitch. He has settled very well in the first few days and we have made him feel welcome as always. It is competition. He is a good goalkeeper and we know a lot about him. We will see how we go.”
Chelsea have done well at bringing players in during the transfer window, with Mendy being the sixth first team signing at combined cost of £220m ($275m).
However, they are finding it harder to sell players with the likes of Emerson Palmieri, Jorginho, Antonio Rudiger, Danny Drinkwater and Tiemoue Bakayoko among those up for sale.
Like those five players, the Blues are finding options for Victor Moses, who spent last season on loan at both Fenerbahce and Internazionale.
Lampard confirmed that Moses, like the other former loanees, are outside the first-team bubble and unavailable for selection ahead of probable moves away from Stamford Bridge.
“At the moment, Victor isn’t in that position with us. We have a bubble at the training ground with the first-team squad, which Victor hasn’t been part of,” he added. “I know Victor well and we will see how that develops over the next week.”