CM - Kyriakos Papadopoulous (if you use a BWM, that is). James Ward-Prowse is another excellent option (better than Barkley at CM, since he is better defensively).
LB - Luke Shaw/ Jonathan Silva/Alex Sandro/Santon (Most likely, Alaba is too expensive).
I dunno many players at LM, but AML - El Shaarawy and Draxler are outstanding. I think you can retrain them to LM too. Also, Vitinho. Brilliant.
RM - Maybe Oxlade Chamberlain (but he'll be too expensive for United). Try getting Lamela and trying to retrain him to RM.