The 28-year-old is back from the knee injury that has forced him to miss three first-team games and has received a firm endorsement from his manager.
"I have no complaints at all," Sir Alex told manutd.com. "I think Dimitar is doing very well for us. His record at international level gives an indication of just how well he has done for his country, but he has scored important goals for us also.
"I have no worries about him. I am happy with his form, I am happy he is with us. I couldn't be happier with him."