The summer of 2021 saw one of the craziest transfer windows in recent years. Not only Lionel Messi left Barcelona after more than two decades, but Cristiano Ronaldo also parted ways with Juventus to rejoin Manchester United.

However, things wouldn't go according to plan for either of them. While the Argentine superstar left much to be desired in his first year at Paris Saint-Germain, the Portuguese striker was unable to succeed in his first season back at Old Trafford. Though Ronaldo was one of the few bright spots of the Red Devils last season, the team couldn't keep up with his level, having its worst Premier League campaign in club history.

If he stays at United, Ronaldo will not participate in the next UEFA Champions League — something that Messi will do as PSG won the Ligue 1 title last term. That could explain why he wants out of Manchester.

Report: UCL goal record over Messi could explain Ronaldo's wish to leave United

Ronaldo, 37, knows he may not have much left in his tank, so every season from now on counts. At this stage of his career, it seems important for him to win as much as possible — which is why United may not be the right place for him at this moment.

His desire to succeed, however, would not be the only factor behind his intention to leave. According to Laurie Whitwell of The Athletic, Cristiano Ronaldo wants to play in the Champions League next season in order to extend his goalscoring record and maybe overtake Messi in group stage goals.

"After discussions, it is felt that a primary objective for Ronaldo wishing to depart is to spend the 2022-23 season at a Champions League club so he can cement his scoring record in the competition," Whitwell wrote. "He currently leads the way on 140 goals — 15 ahead of Lionel Messi, who is two years younger. United are in the Europa League this campaign.

"Ronaldo is also three goals behind Messi in the table of Champions League group stage goals and overtaking his long-term rival on that metric is thought to be an additional objective. Winning trophies and family reasons are also major motivations."

Ronaldo has proven that Father Time has not caught up with him yet, but he knows there is no time to lose. While it's not like he has anything left to prove to anyone, one can understand he will not waste any season left in his career. Therefore, it would make perfect sense to see him leave for a Champions League club in this transfer window.