The whole idea behind U21 teams in this competition is to give those players competitive match experience. We'd look like complete tossers if we parachuted in PL-hardened U21s into our team for the later rounds. Let's just leave it to our U21 squad and see how far they get.
There are 3 more last 16 matches tonight and the last match of the round on Thursday 18th :
Blackpool v Burton
Accrington v Bolton
Peterborough v Crawley
Doncaster v Wigan