Since Montella took over in the end of December the team conceded 5 goals against three opponents (Betis, Eibar, Atletico). Earlier this season Real, Valencia and Spartak all won by big margins as well. Sevilla is occasionally playing well, but overall the team is not in a good spot. The defense is leaky as feck, while the team struggles to be efficiant in the final third. The strongest part of the team is the partnership of Banega and N'Zonzi in midfield; both can play fine football and are very elegant on the ball. The problem is that Correa, Sarabia, Muriel and el Mudo hardly do anything with the ball in the final third. Muriel works hard and is good on the ball, but comical bad infront of the goal. Correa lacks any end-product. Sarabia is Sevilla's lindgard (=motivated, works hard and moves a lot but limited). Vazquez lacks end-product, is very slow and takes way too much time to do anything. Lenglet had a great second game, but he was forced to make plenty of last-ditch efforts to stop United. Thats always the sign of a weak defence. If any team puts pressure on this back4, they'll make mistakes left, right and center. I have seen this sadly way too many times this season.
When Sevilla is allowed to play and enough players have a good day, the team is pretty decent, especially because Banega is a great player. Otherwise things can get ugly (like they already did 6-7 times this season).
United's game-plan and performance against this Sevilla team were atrocious. Its not even that you defended properly. You just relied on Sevilla's weakness in the final third who usually don't know what to do in this area of the pitch. More decisive players could have run havoc with all the space and time. Banega and N'Zonzi were allowed to play their game without much interference, while the defense wasn't really tested. United simply refused to build-up through the midfield. The players were hiding from the ball and struggled to keep/pass it under any pressure. Lukaku played well in the second match; the defenders bounced off him like little boys, but he had no-one to link-up with. It was extremely static. Too many players didn't do anything when not having the ball.
To me it looked like United was not interested in this match. They approached this game like a training match, which allowed Sevilla to cover their worst problems (everybody saw how freaking toothless this attack is). It was a very poor game for this stage of the champions league. United, while not being spectacular by any means, played way better in the past. Two decent performances would have been enough to win against this Sevilla side.For some reasons they didn't show up.