Mkhitaryan will come up against his former club when Roma face United at Old Trafford on Thursday in the first leg of the Europa League semi-finals.
"When Paul was playing everyone was criticising him for his hairstyle. I don't think that has something to do with football, that's his personal life.
"If he plays bad, criticise him for his bad performance, not his hairstyle. That's not good. It has nothing to do with football.
"And you know, some people that don't know him, some people who haven't trained with him or played with him they don't see these kinds of things.
"You have to be in the team, you have to see the way he trains, the way he thinks, the way he suffers and then say if he played good or he played bad."
Mkhitaryan spent 18 months at United before leaving for Arsenal and has fond memories of his time at the club.
"What I want to say is that I had a hard time but after the first two or three months there, I had a very good time."