Oxford midfielder Mitchell Cole has been forced to retire due to his deteriorating heart condition.
The 25-year-old started feeling short of breath while playing back in November - something which was at first put down to general fitness. But medical tests have now identified significant worsening to his heart condition to a level which would put his health in danger should he continue to play.
Mitchell discovered he had the low-risk heart problem when aged 17 at West Ham but it has remained of low concern throughout his career.
He signed for Oxford last summer, passing the club`s medical without any problems, until November when he was struggling for fitness.
The announcement on United`s official website added: "While both the club and player are seriously disappointed at this diagnosis, we are also relieved his deterioration was identified prior to anything more serious happening.
"The club and Mitchell have been working closely with the PFA throughout this process and will come to an agreement in regard to the remainder of his contract in line with FA guidelines in these matters. The club wishes Mitchell the best for the future and will support him fully through this transition."