Although it was reported earlier that the five-time Ballon d'Or winner was getting worried about United's inability to sign new players to rejuvenate their squad, it is now being said that Ronaldo's decision could be because of his eternal rival Lionel Messi.
The 37-year-old football star was United's top scorer last season, having produced 24 goals in 39 matches across the competition, including 18 in the Premier League.
Ronaldo's heroics in 2021-22, however, failed to inspire the Red Devils as they extended their trophy drought to a fifth year and also failed to qualify for next season's Champions League.
And this is perhaps the main reason behind Ronaldo's possible decision to part ways with United.
According to former Aston Villa and Chelsea star Tony Cascarino, the Funchal-born footballer is doing this to prevent Messi from breaking his record of goals in the Champions League.
"Ronaldo is a player, as always, that has an ego. A lot of it is about him but the teams he's played in have been successful, so you always go with that as a team player," Cascarino told British media outlet talkSPORT.
"Great, he scores goals, he gets the accolades, he's one of the greatest that's ever played the game. You can have all that and that's fine, as long as you're winning. When you don't win, now you've got a problem," he added.
With 141 goals to his name, Ronaldo is at present the top scorer in Europe's premier club competition. Messi, with 125 goals in the continental tournament, is only 16 behind him.
But unlike Ronaldo's United, Messi will take part in the 2022-23 Champions League with Paris Saint-Germain (PSG), since the French outfit earnt their place in the event because of their status of being the Ligue 1 champions.
According to Cascarino, the Portugal skipper is doing it all because he wants to protect his Champions League record at any cost.
"I said to you off-air, I'm a bit suspicious because he's got 141 goals in the Champions League. Messi has got 125. He doesn't want to not play Champions League football because he wants to be the greatest Champions League scorer of all time and that's how Ronaldo is made," Cascarino said.