The midfielder is expected to be back in contention after the next international break next month.
Gilmour has stepped up his recovery from a knee operation and is already back in training.
Lampard said: “Billy has been working really well – he’s about three weeks away from returning.
“He’s working on his own with the physio. Hopefully he can integrate with ourselves or under 23s.
“Hopefully it will be two or three weeks. The next side of the international break, all being well.”
Gilmour hasn’t played since July after making a sensational introduction to the Chelsea first team last season.
The 19-year-old has been in recuperation since undergoing surgery, with his recovery estimated to take four months.
Gilmour earned rave reviews after bursting onto the scene, with Roy Keane describing him as “world-class”.
After watching his first start against Everton in March, the Manchester United legend and Sky Sports pundit said: “As the game started I literally got out of my seat, which I very rarely do, and I thought, ‘Who is this kid?’
“There are certain traits you want from a midfielder, quality on the ball, football intelligence, composure.
“He had everything, it was one of the best performances I have seen in a long, long time.
“Just his general play I thought it was like a world-class player. That’s what he looked like straight away.
“I thought it was one of their experienced lads and then I kept a close eye on him for the rest of the game and everything he did throughout the game just had quality written all over it.
“Obviously he’s a small kid but he has a big composure about him and I’m looking forward to seeing him again.”