There was controversy in Poland, where Ajax appeared to be denied a legitimate goal in a 0-0 draw at Legia Warsaw.
Arkadiusz Malarz, the Legia keeper, seemed to pull back Ajax captain Davy Klaassen's eighth-minute shot from behind the line, but none of the officials saw it go in - and there is no goalline technology in the Europa League.
A gloomy fortnight for Celta Vigo continued as they lost 1-0 at home to Shakhtar Donetsk, with Gustavo Blanco scoring the only goal.
The La Liga side had to postpone a home game against Real Madrid just under two weeks ago after their stadium suffered storm damage. They then lost a Copa del Rey semi-final to Alaves before conceding two goals in the last four minutes to fall to a 3-2 league defeat at Atletico Madrid last Sunday.