The goalkeeper was not available for the weekend match at West Brom having joined from Rennes last week.
"Mendy and (Ben) Chilwell are in the squad for tomorrow," Lampard reported from Cobham. "They're fit and we will see whether they start or not.
"(Hakim) Ziyech and (Christian) Pulisic are not far away. They are still in their recovery stage so it's too early for them both."
On Mendy, Lampard continued: "The early signs and the interactions as a man and also on the training pitch have been very positive. He's settled in very well in those few days and we've made him very welcome.
"It's competition, we know that. He's a good goalkeeper, we know a lot about him, and we will see how we go."
Lampard also spoke of Callum Hudson-Odoi's performance for their 3-3 draw at West Brom.
"With Callum, I certainly hope he has a big part to play this season," he added. "When you talk about Ziyech and Pulisic, we're not inundated with wingers because we lost Pedro and Willian in the summer and we need competition.
"If Callum comes on and makes an impact like he did at West Brom and keeps improving every day, he's a huge talent."