Ronaldo was subbed off in the second half of the 3-2 win.
Although he didn't seem irritated for the first substitution in a league game this season, he didn't follow the final minutes with his teammates but instead headed to the dressing room.
"I think it was the first time he was happy to be subbed off. We were one man down, and he'd have been chasing shadows.
"He was happy and smiling in the dressing room," Pirlo said.