McIlroy was ruled out for five months as United struggled to an 18th-place finish in the 22-team First Division.
He later resumed his career, helping the Reds recover from the ignominy of relegation the following season and was part of the side that claimed the FA Cup in 1977, beating Liverpool in the final to deny the Merseysiders a Treble.
After leaving Old Trafford, he played for Stoke, Manchester City, Orgryte in Sweden, Bury, Modling in Austria, Preston North End and Northwich Victoria, before retiring at the age of 38 – a remarkable achievement, given what had happened in 1973 and how long it took for him to recover, as he details:
“I think it happened in January 1973 and the Doc, who had got the job then, tried to get me ready in the middle of May to go on a youth tour to Switzerland which we used to do quite a lot in those days.
“I went, tried to play again but I was nowhere near ready for the following season. With the breathing and my lungs and everything, I couldn’t really get going.
“I went to that youth tour and played a few games but it was nowhere near”