Mullen would become England's first ever substitute, paving the way for many other cameos over the years. Here, the PA news agency looks at his career and the introduction of the rule.
Born and raised in Newcastle, Mullen signed amateur forms for Wolves in 1937. Two years later Mullen made his debut in a 4-1 win over Leeds at the age of 16 years and 43 days, making him officially the club's youngest player, a record that endures to this day.
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Mullen would go on to win 12 England caps, including becoming England's first substitute on this day in 1950. After Jackie Milburn suffered an early injury, Mullen was introduced by manager Walter Winterbottom, and went on to score England's first goal in a 4-1 win.
Mullen remained a one-club man until his retirement in 1960. At that point the Football League was still five years away from its first substitution, when Keith Peacock replaced injured goalkeeper Mike Rose during a match between Charlton and Bolton. Substitutions would not be allowed at a World Cup finals until 1970.
New rules introduced due to the coronavirus crisis allows clubs to make up to five substitutions per match, at the discretion of their respective league. Schalke made the full complement of five changes during their 4-0 behind-closed-doors defeat at Borussia Dortmund this weekend.