The midfielder will join in the summer from Ajax.
"Top clubs will always be looking to improve," said Lampard. "That will be a challenge that's laid down in front of the young players and one they just have to react positively to. It's part and parcel of playing here.
"The young players have made a great impact. We've had a reliance on them to a degree because we knew we couldn't bring anyone in. Throughout the season we knew there were going to be difficult times. You look at Tammy Abraham at the moment and maybe Mason Mount. I think the break was particularly good for them — they can come back and hopefully show more levels in their game.
"They've had tough times recently and it's up to them now to show they are a big part of our plans no matter who we bring in."