United go into the clash with the Spanish side following a morale boosting win over the Reds that put them in pole position to secure second position in the English Premier League behind Manchester City.
Scott McTominay has been deputising in the absence of Herrera, and Mourinho suggested that will continue, so long as the 21-year-old remains humble.
"It's incredible and I have always dreamed of playing here".
The 24-year-old missed Saturday's 2-1 win against bitter rivals Liverpool with an injury sustained on the eve of the game. Speaking ahead of his side's Round of 16-second leg clash with Los Rojiblancos, the Special One feels that United are not among the best teams in the ongoing competition but anything can happen if they arrive in the last eight of the tournament.
With Manchester City proving uncatchable, finishing in the top four is United's only realistic aim in the Premier League. It was the Portuguese manager's fourth major European title and he will be hungry to land the Champions League once more. Or is he just goading Liverpool?
Mourinho clearly would have seen it as a calculated gamble, with Champions League history being largely on United's side as they look to complete the second half of the task by winning at home in the second leg.
"But tomorrow and Saturday against Brighton are the matches where, in the end, you go to the next draw or you don't go to the next draw. So, of course, knockout becomes more important than league matches". And with United having finally made some kind of statement in the league, it's time for something similar in Europe.
United boss Jose Mourinho faces the media later in the day and is sure to be asked about the midfielder's availability - something he was unsure of after beating Liverpool. "I'm really happy with Marcus".