Ifab approved a trial external-link in December 2020 which only allowed for permanent concussion substitutions, although the Professional Footballers' Association has called for temporary substitutions to be brought in as part of the testing. Another first occurred on that opening day of the 1965-66 season when Barrow substitute Bobby Knox scored against Wrexham. Later in the season Knox came on to play as stand-in goalkeeper against Doncaster Rovers and became the first sub to save a penalty. As they could only be used to replace injured players, substitutions were something of a novelty. West Brom, for example, first made a substitution on September 10 but not again until November 27 and used their substitute in only seven of their 42 league games. Their experience was not atypical.

There were games when we were two or three up but Bob didn't even think of bringing me on. It was cowardly. He knew the senior players didn't want to come off. They'd give him trouble but he knew I wouldn't give him any hassle." There are few more self-satisfied sights than the manager whose substitution has just turned a match. Ferguson has said a manager's most important trait is "he must be able to make decisions."During United's 3-2 victory over Aston Villa, Hernandez came on at half-time, shortly before his side went 2-0 down. In the 40 minutes that followed, the striker they call Chicharito had three shots at goal and all three led to goals that turned around a lost match. The referees and the assistant referees are still responsible for the decision on the field of play. A number of managers spoke out against the decision, with Liverpool's Jurgen Klopp and Manchester City's Pep Guardiola both saying the it was partly responsible for a spate of muscular injuries across the top flight. Once this had been conceded, there was pressure for further relaxation of the rules with arguments focusing on substitute goalkeepers. Notwithstanding the efforts of Knox and others, there was little doubt that having to play with a stand-in goalkeeper did disadvantage teams, entertaining though they often were. It was not until 1987 that the rules were changed to allow a second substitute. This proved the thin edge of a wedge with regular rule changes since, starting with allowing two outfield subs plus a goalkeeper, until the current position of allowing up to three of seven nominated substitutes to be used in the Premier and Football Leagues. Already arguments are being advanced in favour of allowing four substitutions.

Bret Myers has tested it against 1,800 fixtures across England, Germany Spain and Italy and believes that if a team is behind, it is vital for the manager to make his first substitution before the 58th minute. However, it has proven to be a popular addition, particularly with managers and players, and IFAB's Football and Technical Advisory Panels have recommended that competitions should be able to decide on an increased number of substitutions "according to the needs of their football environment." Clubs will, however, remain limited in the number of times they can make the changes with a maximum of three intervals allowed plus half time. In light of these high-profile casualties, the FA eventually agreed to experiment with the idea of substitutions. It was begrudging acceptance, as the FA News reported at the time: “The general consensus of opinion would appear to be that, in principle, the use of substitutes in competitive football was not desirable.” The pandemic changed all that in 2020, though. In a move to reduce the impact of fixture congestion caused by Covid-19, world governing body Fifa allowed competition organisers to allow for a maximum of five substitutes.

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Jose Mourinho carved out a reputation for bold decisions during his first season at Chelsea, making 12 double-substitutions and 14 half-time changes on the way to winning the title. Scotland were the opponents for the first ever use of a substitute in international football, when Wales' original goalkeeper failed to arrive in 1889. The issue would need to be raised before a Premier League shareholders meeting, then voted upon by clubs. And, as things stand, there is no meeting scheduled. When it first came in, I thought 'why?' I didn't agree with it at the time, but now it's just become the norm. There's two sides to it - I think it definitely benefits a bigger club with a bigger squad and more strength in depth. You can basically use near enough your whole squad on the bench.