He was taken off at half time presumably because Ampadu wants him and Nketiah (and possibly Malen too) to play for the U18s tomorrow vs Chelsea - where if we don't lose, we will win the U18 league.
I think that fixture will hinge partially on the midfield 2 - Marcus McGuane and Josh DaSilva, who've been very consistent. Having Nketiah and Nelson in front of them gives us a good platform.