The Argentine goalkeeper has started in eight of United's 11 matches in the European competition and will continue on in the quarter-final second leg at Old Trafford on Thursday night.
However, Romero is not necessarily Mourinho's definite Europa gloveman ahead of David De Gea if the Red Devils happen to advance to the semi-finals.
"No, he's not my goalkeeper for the Europa League. He's my goalkeeper for tomorrow," said Mou when asked of Romero's Europa League status.
"He was my goalkeeper in the other matches, but he's not my goalkeeper for the Europa League. I never like to do that complete separation. I can say the priority for Sergio is to play in the Europa League, yes, but we had some matches where David played.
"David's priority is to play in the Premier League but I don't like the complete separation. I cannot say every match is for one or the other. The reality is that Sergio is very stable, he's very experienced, he's playing very well.
"He played in the Premier League [against Sunderland], he played well. He played in the FA Cup, he played well. He played in the League Cup, he played well.
"Every time he plays, he plays well. He's very stable and it's very important to have goalkeepers playing because if not, then one day you could have a problem when one goalkeeper has to play and doesn't have the feeling of the match.
"I always like to rotate my goalkeepers in different competitions but without that complete separation of 'this competition is for me, this competition is for you'."