The Spanish champions were held to a 1-1 draw by United who took a 20th minute lead through a nodded goal from Danny Welbeck before Cristiano Ronaldo responded with a header of his own at the Santiago Bernabeu.
Real had plenty of chances to go a goal in front throughout the contest but were held by a determined United side who weathered the storm and Mourinho now feels that his side has a 50% chance at of moving through to the quarter-finals from the return bout at Old Trafford on March 5.
"It was not a surprise the match or the result," Mourinho told Sky Sports 2.
"I think it was basically what I was waiting for which was everything open, very open for the second leg. I think this will go to the last minute.
"If you ask me the percentage, I don't change one single number, it's 50/50. We can score more than one goal there."
He added: "I think they gave everything. Of course, it was a difficult game because we started strong and had a real big chance when Fabio (Coentrao) hit the post.
"The first time they go in our half, they score a goal out of nothing and psychologically to be losing 1-0 is not an easy situation.
"After that, in the second half they tried everything. It's time to go to Old Trafford. I think 1-1 is not a bad result because everything is open and we can score more than one goal there."