Casemiro reportedly sought alternative medical advice after he was passed fit to play against Liverpool last weekend.
The 32-year-old, who has struggled with injuries this season, ended up being unavailable to play due to an issue that was not picked up by the specialists at Old Trafford. The club have since revealed he is nursing a muscular injury.
The 4-3 win over Liverpool in the FA Cup was the 17th game he missed for United due to injury this campaign. According to Brazilian news outlet Trivela, the midfielder went to medical staff with a hamstring complaint leading up to the match.
However, checks that were made by the medical staff did not detect any significant problems with Casemiro and he was passed fit to play. He is claimed to have ignored their advice and visited a doctor in Barcelona.
Trivela adds that a 'small issue' was discovered in his hamstring - the same hamstring that kept him out for three months earlier this season. It is believed that the issue could have flared up into something more serious if he had featured against Liverpool.
Having to withdraw from the Brazil national team, outlet Jogada 10 claims that Casemiro drove three hours from Manchester to Saint Albans to meet up with Dorival Junior and the rest of the Brazil squad in London this week ahead of their match with England on Saturday. The report adds that he was reassessed by Brazil doctor Rodrigo Lasmar, in whom he places great trust, before withdrawing.
Speaking about Casemiro's absence, Erik ten Hag said: "It does [affect us] because Casemiro is very experienced and a very important player for us in organisation, also on the ball. He brings composure to the team and he can bring great passes to bring our frontline into great positions, especially with the three with their immense speed. You miss, of course, a passer like Casemiro."
The Brazilian will have the chance to rest up during the international break with United next in action on March 30 at Brentford (8pm).
MEN Sport says: Whichever way you look at this incident, it does not make good reading for United's medical department. There is currently an increased amount of scrutiny as to why the Reds have managed to pick up so many injuries this campaign. You cannot doubt that the increase of matches has had a bearing on the troubles but these reports, if true, will undermine faith in the medical department further. Another injury to Casemiro also spells bad news for his United career. Reports suggest Joao Gomes is a target for the summer to replace the Brazilian and it is already clear Sir Jim Ratcliffe is not a huge fan of the transfer, add his unreliable injury record and it could spell the beginning of the end for him at Old Trafford.