Barcelona's only chance is that United will treat the Europa League as their second priority this season. There are all prerequisites for this - United are doing well in the Premier League and they have every chance of not only gaining a foothold in the top 4 but even climbing higher. Given the value of qualifying for the Champions League, this looks like a bigger deal than beating Barcelona. Winning the Europa League also gives a direct ticket to the Champions League but it is less "guaranteed" as there is a lot of randomness in the knockout games.
I think that Barcelona has the better chances here but the fact that they are directly fighting with Real Madrid to win the La Liga makes it a bit more difficult for them compared to Manchester United which has no title winning objective and it is impossible for them to win the title in the Premier League.
Barcelona when compared player by player I think has a better quality and normally should qualify further in the Europa League and be a strong contender to win the Europa League but in these encounters you never know how it might go because it is just a 2 game elimination so it can be anyone's game.
I'm not at all sure that in terms of players, Barcelona will be ahead, but it doesn't matter - football is a team game and United look much stronger here. United are more flexible and they are able to play with different opponents using different strategies. I don't follow Barcelona much but their game looks like either stupid ball rolling or crosses on Lewandowski "for good luck" with extremely low efficiency. Is there any chance Xavi will come up with something new? I highly doubt it.