The Red Devils went behind early in Belgium thanks to Emmanuel Dennis' lob over Sergio Romero.
But Anthony Martial scored his second in as many games to level the tie.
That goal will count for two when the sides meet again at Old Trafford on February 27.
Speaking after the game, Juan Mata said: "They started well, they play good with the ball and defend compact.
"We shouldn't have concede but we scored from their mistake.
"Second half we played much better than the first. We had some chances."