Pulisic came on for his Premier League debut in the second half of Chelsea's 4-0 loss to Manchester United on Sunday.
Afterward, Lampard explained his decision not to start the United States international, who arrived in the close season after being signed from Borussia Dortmund in January for a reported £58million ($70m).
"Christian Pulisic is 20 years old. He will be a fantastic player for the club. He has got great ability. You saw little moments with his acceleration on the ball," he said.
"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, 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."