Most successful sports people can recall one crossroads in a career, an event that made the difference between turning professional and reciting the “could have been a contender” speech.

For Liverpool’s new £10 million left-back Andy Robertson, there is already a catalogue of such defining moments.

At 15, Robertson was told by Celtic he was too small and timid to play at the highest level, a crushing disappointment he says inspired him to prove the sceptics wrong.

By 18, now working in the corporate department at Hampden Park while playing amateur football for Queen’s Park, Robertson was given a deadline by his supportive but realistic parents; secure a professional contract in the next 12 months or consider alternatives.

Whatever challenges face him at Anfield, Robertson is used to playing for the highest stakes.

Robertson is on pre-season tour with Liverpool in Germany

“For the first few years when I was in the youth side at Queen’s Park it was fine because I was still at school,” says Robertson.

“I was grateful to my mum and dad because [after leaving school] they said, ‘We will give you this season to try and push on and make that dream a reality, but after that you might have to look at other options’. I was going down the line of needing to apply for university or college or maybe becoming a PE teacher or something in sports science. Queen’s Park was amateur so you do not get paid. You need to make a living with that.

“I worked in Hampden Park taking phone calls and [ticket] orders for games. We trained twice a week at night and played games on Saturday, so I was working 9 to 5 and then having to train.

“There was a guy, Andy McGlennan, who had worked at Queen’s Park for years and he was high up at Hampden so sorted a lot of the boys out with jobs. He looked after us and made sure I had some money in my pocket with a bit of hard work. He knew what the dream was and helped me.”

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Rejection by Celtic, Robertson’s boyhood side who as a fan he watched knock Liverpool out of the 2003 Uefa Cup (his father grew up worshipping Kenny Dalglish), left a scar. “There was a transition going on at Celtic at the time where a new head of youth had come in. I didn’t fit the bill,” said Robertson.

“He came from Motherwell, who were full of big lads and were physical. That wasn’t my game. I was small. I’m not big now but it took me time to grow and fill out.

“He saw a small guy playing centre-mid, left-mid or left-back who was quite weak. To be fair I was, but I believed in my ability.

“That was the first time I had anyone doubt me. Tommy Burns [the former Celtic manager and head of youth development] was great with me and when he died it hit everyone hard. He was different class with me, he liked me as a player, he liked me as a person and he could see what potential I had.

“He sadly passed away and Celtic went another route afterwards, one which was hard on some players, but good for others that maybe didn’t fit Tommy’s vision.

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“Being rejected was quite hard because I was a Celtic fan from birth. It took nearly a full season to get over that disappointment. My first year at Queen’s Park, I just wasn’t good enough, but that tough period shaped me.

“I’ve had a few doubters since then, but you’ve just got to continuously prove them wrong because if you are, it means you’re doing things right. Looking back on it now, being released was the best thing that could have happened to me.”

So has there been any contact from those at Celtic’s academy who made such an expensive mistake?

“I don’t want them being in touch just because of what’s happened now,” he says in the luxurious surrounding of Liverpool’s German training camp in Rottach-Egern.

“Would they be in touch if it worked out the other way and I was a PE teacher somewhere? No.”

Robertson impressed Jurgen Klopp with his performances as Hull credit: Getty Images

Robertson could not have imagined it was his response to this childhood setback, thriving at Dundee United and Hull City, that caught Liverpool manager Jurgen Klopp’s attention as much as his stylish full-back performances.

Klopp empathised with what he described as the 23-year-old’s “incredible personal journey”.

“The manager wants to know the person as well as he knows the footballer,” said Robertson. “He was asking me about my story. He told me about himself and wanted to know all about me.

“I saw the quotes when I signed for Liverpool and he obviously liked my journey from the bottom of Scottish football to where I am now. It has driven me along.”

The Anfield interest in Robertson pre-dates Klopp – he was first under consideration two summers ago – but he left an impression on the German coach when shining for Hull last season.

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Robertson lived with the rumours of a Liverpool bid for a while, but said he only felt sure of their interest once an offer was made. “It’s hard because there are scouts at every game. They can tell your agent or whoever they are looking at you, but you never really know,” he said.

“I’m sure they go there with an open mind and it is about who catches their eye. That’s the nature of football. You don’t take too much attention until there is a formal bid. There was a lot of speculation about it but that’s football. You just need to get your head down, work hard and make it a reality.

“When Liverpool come in it’s a no-brainer and with a manager like here, the first time I spoke to him I was taken away by him and his plans. Obviously I’ve played against his side and know how hard it is against Jurgen Klopp’s team. I want to be part of that team and make it hard for teams to break us down and beat us. A team like Liverpool you are not going to turn down. I couldn’t wait to get here once the fee was agreed.”

Robertson will have an immediate chance to impress during Liverpool’s German tour due to a thigh injury to James Milner, who will not feature in the three games. Many believe Roberston will initially be Milner’s deputy, but the youngster has loftier ambitions.

Left-back has been a problem position for Liverpool credit: Getty Images

“I’ve come here wanting to be first choice,” he said. “Obviously it is up to the manager as the competition will be big for the position, but it’s one where I’m hoping to come out on top. I don’t like it if I’m not part of the starting team. I don’t like sitting on the bench even when it was very rare at Hull to rest during cup games. I’m not a good spectator, I’m a nightmare when I’m injured and I’m not playing.

“I respect the other players and I know how good a job they can do but I’ve just got to bring my A-game and hopefully it’s enough for me to first choice.”

With rival Premier League clubs paying £50 million for overlapping full-backs, Robertson could become one of this summer’s bargains. He laughs at the thought of a £10 million purchase being seen as such.

“I wouldn’t quite say that,” he said. “We know over the last few years that football’s been a bit crazy. The investment all around the world that is making people spend kind of silly money. I think it’s going to keep going up and up.

“You don’t focus on the price tag, you just try to focus on when you get there. The players, themselves, can’t control that. If you’re a wee bit more expensive then there’s maybe more pressure on you, but when you come to big clubs like this one, there is always pressure on you.”