Ronaldo left United and joined Real Madrid for a world record fee in the summer, but still watches every United game on TV.
The former Sporting Lisbon starlet believes Nani can fill the void left by his Old Trafford departure.
“Every time I can I try to watch United’s games on television,” said Ronaldo.
“I still like to know what’s happening at Old Trafford. And despite now being at Real, I’m always happy when I know when United have won.
“I’ve also seen that Nani is playing much more than he was last season when I was there. That’s good for him and he will benefit from that.
“Now he has more opportunities to play, he will be able to show the fans just how a great player he is.
“We still speak regularly on the phone. And it’s good to see United’s fans are starting to appreciate him more and have been happy with his form up until now.
“Nani is an excellent player. There’s a lot more to come from him and I’m sure that in a few years he will be amongst the best players in the world.”
