Ole Gunnar Solskjaer's side are on the cusp of this month's Gdansk showpiece having secured a 6-2 win in last week's semi-final first leg at Old Trafford.
The Red Devils are overriding favourites to progress past Roma, who sent ripples around the footballing world on Tuesday by announcing that Fonseca will be replaced by ex-United boss Mourinho next season.
"For me, professionalism is something I believe is very important," the outgoing Giallorossi boss said on Wednesday.
"I am OK, I am focused on things here just like I was the very first day I got here.
"I am motivated. I want to do the best I can for Roma for as long as I am still here.
"Mourinho is a great coach, everyone knows that, and I am sure he will do a great job here."
"Nothing is impossible. I believe that anything can happen.
"I'm expecting a tough game. Going up against Manchester United is always very difficult.
"They have a good first-leg advantage but we want to win the game. We want to fight until the very end."