Amid deeply-worrying scenes, the Scottish international went down apparently unchallenged, with his team-mates quickly indicating for the medics to run on the pitch to tend to him.
Fleck received “urgent medical care” according to United’s official Twitter account and, after a 10-minute delay, was taken off on a stretcher and out of the ground to a waiting ambulance.
United later stated on Twitter the player was conscious when he left the stadium in Reading to make the short trip to the Royal Berkshire Hospital, where he stayed overnight.
A United statement on Wednesday morning said: "Sheffield United is grateful to report that John Fleck has been discharged from hospital and will return to Sheffield today (Wednesday).
"He was conscious when he was taken to the Royal Berkshire Hospital and was communicative with club and medical personnel, as well as his family.
"After comprehensive medical examinations and staying in hospital overnight for observation, he has been allowed home, where United medical staff will continue to monitor him closely.
"United and John Fleck would like to thank all who assisted at a stressful time last night and also everyone for their supportive messages."