The midfielder has been out of action for four months after he was diagnosed with a rare eye condition, the paralysis of the optic nerve, which affected his vision.
"It was tough staying off the field for that long. Now I just need to improve my fitness and my position on the pitch," he said after making his comeback in Milan's 1-0 win over Paris Saint-Germain.
"The most difficult thing in all these months was facing the everyday challenges. It's not easy to get up in the morning when you're seeing triple or even quadruple.
"My family and friends gave me the reassurance to carry on and were always close. Milan are like my family. I've done good and less good things in my career, but the club has always helped me out."