If Barcelona plays the same way against United in the match against Madrid, then United will have a very difficult time. Both teams seem to be doing well. Perhaps this will be the most anticipated match, perhaps even more than the PSG - Munich match in the CL.
The Europa league will always been a lower competition, no matter the teams that are playing in it, i'm sorry this game can't be anticipated than any match in the champions league round of 16. Barcelona and United are not even going to play at a level they would have played if this game was in the champions league. I believe United are going to win, they started the season in the Europa league, so i can say they are now used to it, but barcelona may still be disappointed that they were eliminated from the champions league group stage, so the disappointment may still be in their mind. Barcelona also have a big chance to win the La Liga, so that is where most of their focus will be in.