Ole Gunna Solskjaer was annoyed with Manchester United's discipline, despite their 2-0 win over Granada on Thursday.
The Red Devils secured two crucial away goals thanks to Marcus Rashford and Bruno Fernandes.
But they will be without Harry Maguire, Scott McTominay and Luke Shaw for the second leg after the trio picked up yellow cards at the Los Cármenes.
"It was not a perfect night," Solskjaer said after the game. "We got three yellow cards and three suspensions. 2-0 is very good result. We know how difficult it is to come to Spain. We've got to play well to get a result.
"They [Marcus Rashford and Bruno Fernandes] have been exceptional, so important for us. It was a very good run by Rashford, takes the ball fantastically. Bruno is so confident on penalties even though the keeper almost saved it.
"Same as always [attitude for next week]. We want to win every game. It's still a young team, a learning team. We have to improve all the time. Football catches you so quickly if you rest. We can't have any other approach."