Speaking to BT Sport, Ralf Rangnick said: "The second goal which was disallowed, I watched it in the locker room.
"The linesman flagged foul but he flagged it ten seconds after the incident. It was a soft foul.
"We played an excellent first half and then in the first 15 minutes [of the second half] we were not aggressive enough.
"That is why we gave away the goal. The way that we allowed them to score, we did not defend the situation well.
"In the end it was also a little question of luck. One point for a performance like this is just not enough."
Manchester United actually had two goals disallowed in first half in the 1–1 draw away at Burnley in the Premier League on Saturday.
The first one, scored by Raphael Varane from a set piece, was called off due to Harry Maguire allegedly impacting the play from an offside position.
The second own goal, put into his own net by Ben Mee and mentioned by Rangnick, was called off because of an alleged foul by Paul Pogba during the build-up.
