Ronaldo scored United’s first goal, but was left unhappy with being substituted earlier in the second half.
Fergie told MUTV: “He wanted to stay on, that’s the simple reason. He’s in great form, but I have got to look at the bigger picture. He had an incredible performance at Arsenal in midweek, he ran his socks off. We have to think about how we get through our games and keeping him fresh.
“It’s great when players want to play. Take Jonny Evans, who was taken ill this morning. We put him on medication but he still said he wanted to play. He was feeling his hamstring at half time but again he wanted to play on. That’s the guts you want. I brought him off because I can’t take chances at this stage of the season.”
Sky Sports News claims the Portugal international left Old Trafford just 10 minutes after the final whistle.