Exactly, this is what makes me a little doubtful of Xavi. because so far the strategy has only been proven in La Liga. We look at the results of Barcelona's match in the UCL, he was only able to win against Viktoria Plzen. Even though MU is in inconsistent performance, if Xavi's strategy is limited to the La-Liga level, there is still a chance for MU to beat them. Luckily, in this match Barcelona have advantages because Ronaldo isn't in MU squad anymore. 
Both teams are currently in good form in my opinion, Barcelona managed to rank first in La Liga, while Manchester United is growing and becoming more consistent until the second half of the season, even though they last lost to Arsenal. However, if you look at the teams that often win the Europa League, both of them are not strong candidates, so it is difficult to predict the champions. The only first leg advantage is due to playing at Camp Nou, so Barcelona's chances are bigger than Manchester United.Head to head is also still quite superior to Barcelona compared to Manchester United for now, although the final result is also not determined by these two factors, while the quality of coach Erik Ten Hak is superior to Xavi based on their previous achievements. In my opinion, many factors are taken into consideration by the two teams and who will qualify for the next round.
MU has been in good form and the qualities of the players are increasing day and the team has a good defence,which makes this match unpredictable yet until on the match day,when these two clubs will be on the field playing.