I think Barcelona have more chances of winning Manchester united. Barcelona has Lewandowski who can score some number of goals, this is an advantage for Barcelona. The game will be interesting, this are the big clubs in the Europa league.
If it's based on the history of head to head between MU and barcelona and i'd say that barcelona was leading the way over MU caused by it has so much winnings against MU but with the current situation and i doubt that about barcelona will able to beat MU. It caused by MU will get some great players as new players to play against barcelona. Barcelona must be careful with MU. Barcelona is likely trying to sell de jong for MU which can be considered as a boomerang to the team. These big clubs were making europe league become even better.
MU may give a huge pressure for barcelona but the match will come in february which is still very long and im sure barcelona will also change its starting line up