The Red Devils are rocked by the news that Ronaldo wants to leave for a team in the Champions League.
According to British press reports, Chelsea are the most likely to offer him a route away from the Old Trafford club.
Ronaldo did not turn up for pre-season training on Monday, citing family reasons for his absence.
According to The Athletic, a source close to the team has said that the United squad would be "liberated" if Ronaldo left.
United finished sixth last term with Ronaldo, while they were second in the league the year before his arrival.