World Cup-winner Desailly, who played alongside Terry at the heart of Chelsea's defence for almost four seasons, insists Grant has to find a place for the 27-year-old.
"John Terry, he is the man - he has to play, definitely," the former France star said on BBC's Football Focus.
"He's important on and off the field."
Terry and Lampard have only recently returned from around two months out injured, and Chelsea have coped extremely well in their absence, with the likes of Brazil defender Alex and Germany captain Michael Ballack coming to the fore.
Despite championing Terry's cause, Desailly can see Grant's logic in omitting Lampard.
He said: "He has an idea on Frank Lampard. He wants to play another player maybe who is a bit more fit than him. Maybe, the way he set up his team, he is thinking of leaving out Lampard."
Desailly believes Grant is deliberately keeping his players guessing over the starting XI.
"There's bit of polemic on which players he's going to select," he said. "But, don't worry, he is doing that trick to push the players and make them feel a bit more concerned."
"I'm surprised because I think they are two influential players in the team," he said. "But I'm sure that Avram Grant knows these two well so he must have a good reason."