Louis Reed became United's youngest ever player in a league game on Tuesday night.
At the age of 16 years and 257 days, Reed was introduced as a second half substitute in the 1-0 derby victory over Rotherham United.
Club statistician Andrew Kirkham has scoured the record books and confirms that Reed beats all peacetime records - only Dennis Thompson, in a wartime game, was younger.
Before Reed's debut on Tuesday, Stephen Hawes (17 years and 47 days) was the youngest, also making his debut as a substitute in 1995.
Thompson was 16 years and 103 days when he made his debut in a wartime game.
Other 16-year-olds to make their debut include Trenton Wiggan (16 and 329 days) when he was a substitute in a League Cup game against Doncaster in August 1979, whilst leading appearance maker Joe Shaw was 16 years and 283 days when he made his debut in a wartime game.