The Argentine has started the previous 10 matches in the competition, managing to keep six clean sheets and despite suggestions that Mourinho might start de Gea, Romero will keep his place.
"They are two fantastic goalkeepers. I never saw, in all my career, two goalkeepers be such friends," he said after Thursday's 1-1 draw at Old Trafford, which saw United scrape into the final to face Ajax.
"You always have a little bit of rivalry, especially if you are both the same kind of level. We're speaking about the Argentina national goalkeeper and Spain national goalkeeper.
"They support each other all the time. I never saw a bad face, I only saw them support each other. I think it's fair that Sergio plays the final and David accepts that. Especially because he played two Europa League matches, so if we win, he wins the trophy.
"If everything goes normal and there are no problems, Sergio plays the final."
United's preparations have taken a hit, with defender Eric Bailly's red card in the closing stages against Celta meaning he'll miss the match but Mourinho says there is little point in appealing the decision.
"I don't know the incident, I don't want to see it now," he said. "Even if it was unfair, I don't think the rules will allow any chance of appeal. So I don't think he plays the final, so he has to play the remaining matches in the Premier League and give rest to some of the others."