It was the tactical decision by ETH that cost Man United the game. Like you said Martial was having the best of game and he was replaced with the ineffective Weghorst.
~snip~
It looks like the draw at Old Traford benefited Sevilla a lot. Their second leg will be played in Spain. Many players are injured and accumulated cards, hoping that Man United can qualify seems very difficult.
As the host of the second leg, of course Sevilla is very lucky. In fact, I also read that Rashford is also unavailable in the second leg. It is possible that Sevilla will continue the record of never losing and qualifying for the semi-finals.