Ronaldo is also understood to be concerned that United are yet to make any improvements to the squad in the current transfer window. Thirteen players have left the club this summer but none have yet arrived.
Ronaldo respects United a lot - especially the fans - and is said to be "in love" with the club, but this coming season will be the first in his career in which he will not play Champions League football if he stays at Old Trafford.
United finished sixth in the Premier League, missing out on Champions League qualification, leaving the five-time Ballon d'Or winner, who has a year left on his contract plus an optional year, facing the prospect of playing in the Europa League for the first time.
Manchester United are adamant Ronaldo, who scored 24 goals in all competitions, is not for sale, according to reports, with new manager Erik ten Hag keen to work with him.
Ronaldo won three Premier League titles and the Champions League during six trophy-laden years with United before leaving for Real Madrid in 2009 and Juventus in 2018, collecting more trophies at both clubs.
United's international players are due to report for training in the coming days ahead of flying to Thailand on Friday for the start of their pre-season tour.
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