The 22-year-old was the Portuguese’s first signing as United boss, joining in a deal from Villarreal which could cost up to £25 million.
Bailly had played just 35 games in La Liga before his move to United.
And while Mourinho believes the centre-back has the quality to become a success, he is unsure whether the Ivory Coast international can instantly adapt.
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“I could compare him with some players I had previously but I don’t want to do it because the boys play for other clubs," the United manager told MUTV.
"He’s a player we believe has all the attributes. Very strong physically, he's very fast. It’s not so easy to find a fast player when you are quite a big body, a heavy guy, a tall boy.
"He comes from a Spanish culture where the first building-up phases are important, so technically he's very good.
"In some periods he played a right-back and because he played as a right-back which demands more by the attacking point of view, so it's for a central defender to play as a right-back.
"My only question mark is a question mark I have to help him delete it, which is a question mark for somebody who has never played for such a big club, with so many responsibilities, with so many expectations.
"So is Eric ready to come here and start performing from day one? I don't know. It's a question mark, my job is to delete the question mark as soon as possible, and to make him a player with stature, brain and personality to play for Manchester United."