There is a story Kevin Keegan tells in his autobiography about Bill Shankly, his manager at Liverpool in the early 1970s, concerning a psychological ploy effected by the Scot before an encounter with West Ham United and Bobby Moore.

The great Moore, Shankly claimed, had been “out in a nightclub again” and Keegan would “run him silly”. Ninety minutes later, after Keegan had scored the only goal of the game, Shankly sang a deliberately different tune, affirming: “You’ll never play against a better or fitter player… but you had the beating of him.”

As this tale underlines, if football is a sport played on a rectangle of green, a key element of it takes place in the head. Shankly, the architect of Liverpool’s first-ever European success, in the 1973 UEFA Cup, knew this as well as anyone and earned a reputation for his mastery of kidology. Recent history tells us Jürgen Klopp, Anfield’s current incumbent, has a strong appreciation of it too. As José Mourinho observed of Liverpool’s epic come back against Barcelona last season: “This is about heart and soul and a fantastic empathy that he creates with that group of players.”

Klopp’s powers of motivation took Liverpool all the way but the Reds were not the only team to defy the odds in the competition last term; indeed, Tottenham Hotspur’s own miraculous recovery of a three-goal deficit atAjax 24 hours after Liverpool’s fightback raised the question whether players are now conditioned to believe no result is beyond them.

It is a question that Danny Donachie can answer better than most. Currently director of medical services at Premier League side Everton, he has worked across a range of sports – football, tennis and boxing – to enable athletes to unlock that extra one or two per cent. “To help the player be more present to their reality and more present in the game is a huge thing,” he says.

In the case of Champions League comebacks, Donachie cites the Pygmalion Effect – a case study in a Californian elementary school in the late 1960s that showed how teachers’ expectations influence pupil performance. In the football context, players draw encouragement from previous reversals of fortune. “Unconsciously we take things in all the time, so if a player sees another result happening like that in unlikely circumstances then unconsciously we take in the belief that it could happen again,” explains Donachie.

“With the Champions League this has happened a few times now, so teams are going to see that and believe. As soon as the other team scores a goal, the team who are winning have it in their unconscious as well, so it’s a two-way thing and the dynamic can change in a minute, as you’ve seen.” The effect is stronger still when teams have direct experiences to draw on – as in the Liverpool-Barcelona tie where the home side had their history of past comebacks, and the visitors a recent collapse against Roma.

For Donachie, the key for any sports person is focus – and the capacity to clear the mind. “If you do a bad thing, you could be thinking about it for the rest of the game,” he says. “There’s no way you can be fully present if you’re thinking, as thinking takes you away from the present. Players at their best are intuitive, fully present and not thinking.”