Although 1-0 up against a side who had been reduced to 10 men, the Italian champions eventually suffered defeat after Mauro Icardi's second-half double.
"I was still moving my feet back when Icardi got his shot away, leaving me off balance," the 34-year-old commented.
"I don't usually commit such errors, but this time I was too far forward. I'm sorry because my error compromised three points which seemed almost certain.
"The second goal was different because I would have had to pull off a truly special save to keep it out. Having said that, I wouldn't have been surprised if I'd had managed to do so."