With the Blues currently serving a transfer ban, Pulisic is the only new signing at Chelsea this summer after his £58 million ($73m) switch from Borussia Dortmund was agreed in January.
Lampard opted against handing Pulisic a start in his first competitive match in charge of the club on Sunday, choosing instead to field Pedro and Ross Barkley in the wide areas with England youngster Mason Mount in support of forward Tammy Abraham.
Pulisic eventually did make it onto the field in the 58th minute when he replaced England international Barkley. Chelsea were already 1-0 down at that stage courtesy of a first half penalty from Marcus Rashford.
Pulisic had little time to make an impact on the game before United doubled their lead through Anthony Martial on 66 minutes.
Further breakaway goals from Rashford and debutant Daniel James completed a sobering afternoon for Chelsea at Old Trafford, with the final 4-0 scoreline the heaviest ever opening-day league defeat in the club’s 104-year history.
Speaking after the game Lampard revealed his reasons behind his team selection and his plans for Pulisic going forward.
“Christian Pulisic is 20 years old and he’s come across from Dortmund. He will be a fantastic player for the club. He has got great ability. You saw little moments with his acceleration on the ball,” Lampard told reporters.
“But I have also got other options in midfield and in those areas, and I don’t want to stick Christian straight into the fire without trying to help him along the way. It was one of those games where I felt it was better to start with Ross [Barkley] and Mason [Mount], who have been very strong in pre-season, and bring Christian on.
“If the game had carried on at 1-0, or we were back in the game as we should have been, he would have been a great injection. In the end it was difficult for him because the game went quickly away from us.”