United States international Pulisic hasn't played in four of his club's last six matches and has spoken of his frustration at not featuring as much as he wants to. He was left out of Chelsea's squad completely for the Champions League win away at Lille and was named on the bench for Sunday's match at Southampton before coming on for the final 10 minutes.
Lampard said: "Sometimes I have to make harsh decisions and say something that I see can help their game but might not sound that nice because I want them to be the best, all of them. That's what I want them to be. That's part of the process.
"Christian Pulisic has a price tag. Ross Barkley is an international player. Mason Mount is an international player, Callum Hudson-Odoi just signed a new contract, Bayern Munich wanted to buy him last summer. He's an international player.
"We can all focus on Pulisic from the other day but Callum's the same and I have to speak in the real world when I speak to them and sometimes as a manager you want more.
"You have to be honest and the players have to react to that and Callum has now come on with two assists in the games that he's come on and that's what you want to keep pushing. I love the young players. I particularly love the young players because I want them to improve daily."
Pulisic, 21, signed for Chelsea in January and was immediately loaned back to Borussia Dortmund for the remainder of last season. He did not feature for Lampard in the Premier League at all in September, with his last appearance coming in the League Cup against Grimsby.
He has been named in United States national team manager Gregg Berhalter's squad for the Nations League matches against Cuba and Canada.
On the player's difficulties since moving to England, Berhalter said: "This is what happens when you transition into a new team, to new leagues, and it's a common issues that we see. He's going to be just fine. For us, his status is the same as it's always been."