Chelsea decided to ditch Matic, 29, after completing the protracted signing of France international Tiemoue Bakayoko from Monaco.
The Premier League champions allowed him to head to Old Trafford as a goodwill gesture for his contribution to two Premier League title wins.
But Lampard is unsure of the decision to replace Matic for Bakayoko in the heart of midfield.
“It's a strange one,” the Stamford Bridge legend said on BT Sport.
“Such a huge part of Chelsea's play last year was Kante and Matic being the base in centre-midfield.
“It let the wing-backs fly on, it let Pedro and Willian and Hazard have freedom to go forward.
“Fabregas was a big player for them last year, sometimes as an impact player.
“Bakayako, it remains to be seen if he can be that positional player that Matic was.
“I think one of the frustrations will be that and he'll want to bring someone else in in midfield.”
Diego Costa - another key component in last season’s title-winning team - has been frozen out by Antonio Conte but Lampard believes £75m-replacement Alvaro Morata is a top signing.
He said: “Morata, I think he'll score 20 goal. Diego Costa scored 20 goals last year and I think Morata will. That's why he's there.”
