Lawrence was previously in charge of our Under-18s side, leading the youngsters to the national title in 2023/24, in addition to the Premier League Cup, before taking up a role at Newcastle United.
Darren Fletcher remains in charge of the Under-18s.
The Under-21s lead coach role was vacated when Travis Binnion joined the first-team coaching set-up earlier this month.
Darren Fletcher continues as Under-18s lead coach and will now also take on additional responsibilities by leading the Professional Development Phase (Under-17s-Under-21s) for the remainder of 2025/26.
The Under-21s are next in action on Monday night, facing an away game in Premier League 2 at West Bromwich Albion.
Meanwhile, the Under-18s, fresh from a dramatic late win at Burnley, shown live on MUTV, travel to Middlesbrough on Saturday 7 February.