It is with matter-of-fact delivery, rather than obvious emotion or regret, that Matija Nastasic recounts the meeting that in effect brought about his departure from Manchester City. He had made a rapid impact after arriving from Fiorentina as a 19-year-old in 2012 but, two and a half years on, had fallen off Manuel Pellegrini’s radar with no obvious explanation bar some niggling injuries he had long since overcome.

“I was thinking a lot: ‘What is the problem?’” he says. “After some time I spoke with Pellegrini and he explained I had to wait for my chance. In the end there were not so many opportunities to play. There was no issue between us but I couldn’t understand why it was like that and told him that I felt I could help the team more, that I should have more playing time at that moment and that, if it was not possible at City, then it would be better for everybody that I changed club.”

Nastasic now has a chance to show City what could have been and, while it is hard to make a case they have missed him given the strides made under Pep Guardiola, a fair assessment is that he would not look out of place in today’s squad. Instead he will play for Schalke against his old employers at the Veltins Arena; he is 25 and well established as one of Europe’s most reliable all-round central defenders.

“To be honest I didn’t want this tie at first,” he says, recalling how his teammates’ eyes turned to him as they watched the last-16 draw here, at Schalke’s training ground. “Everybody knows how strong City are. But on the other hand I was happy I’d be going back to see some old faces. It will be an honour to play there again.”

When Nastasic joined City he hardly spoke English and relied on his fellow Serb Aleksandar Kolarov and the Bosnian Edin Dzeko for help early on. On the pitch he adapted with ease. A debut at the Bernabéu, anchoring the defence alongside Vincent Kompany, was the first of 30 appearances in 2012-13 and, while the season ended with an FA Cup final defeat by Wigan, few of his peers emerged with their reputations similarly burnished.

“The first season in Manchester was great,” he says. “I’d felt ready to go there and make the step, because maybe in life you have only one opportunity like this. When I arrived I felt everyone’s support and it was as if I’d been there for a couple of years already. I played almost every game, I was fit all the time, so it was good.”

He still cherishes the influence of Kompany. “I learned a lot from him. He is such a professional and a great man who talks a lot with the players, especially the young ones. It was very important to play alongside him; I’m really happy to see he’s still there because he’s a legend, really. In the dressing room he kept the atmosphere at the highest possible level.”

It was a fine education and Nastasic also benefited from living in the same neighbourhood as Nemanja Vidic, another compatriot and childhood idol – “the type of player I wanted to be one day” – who gave his time generously in those early days.

Matija Nastasic in his Manchester City days. Photograph: Laurence Griffiths/The FA via Getty Images

Nastasic had blossomed under Roberto Mancini’s management but life with Pellegrini was different. Knee and calf injuries disrupted the title-winning 2013-14 season and errors crept in. He missed its final three months and there was no way back to the extent that Pellegrini, listing his centre-backs in October 2014, failed to name Nastasic until prompted by a journalist.

“It was a little shock for me,” he says. “You get one injury and after that you aren’t playing that much. I was young, and at that age you think: ‘I have to play all the time’ and all these things. When I look back now it was not easy for the coach; we had a lot of good players, as every big team does. Things were up and down but I was really happy we won the title and that I can say I was part of it.”

The admission of youthful impetuosity should not be confused with self-reproach about leaving for Schalke, who he joined – initially on loan – early in 2015 after being impressed by their then manager, Roberto Di Matteo. Another 18 months would pass before Guardiola’s arrival at City but, in a parallel timeline, would he have liked to try his luck under the Catalan?

“Yeah, when I see how they play now and the style of football – a lot of dynamic pressing and possession – I think every player would like to play under a coach like him. It’s really unbelievable to see and I can’t imagine how it is to train every day and do things like that. But I spoke to Kolarov and he told me that he’s the best coach he had. Kolarov has had a lot of coaches so if he says it, that’s how it is.”

Pretenders to Guardiola’s crown are emerging and among them is Domenico Tedesco, the gifted 33-year-old whom Nastasic believes has transformed him at Schalke. “I think I especially improved in the last two seasons under Tedesco,” he says. “He’s a great man for this club, really unbelievable on tactical things. His type of work is with a lot of aggression, a lot of tactics and, the most important thing for me, a lot of talk with the players.”

Perhaps a little more of that from Pellegrini would have benefited Nastasic at City. But he is a pillar of Schalke’s dressing room now, and those with an ear to the inside say he is an intelligent, increasingly influential voice in a squad that finished second in last season’s Bundesliga but lost a leader upon the summer departure of Leon Goretzka for Bayern Munich.

“I started very young and, especially as a defender, it’s hard to be on the highest level [at that age],” he says. “But now I feel great and think the best years are ahead of me.” He says he wants to help nurture Schalke’s next generation, and one of them is the winger Rabbi Matondo, who joined from City in January and made his first Bundesliga start against Freiburg on Saturday. Matondo’s maturity has made an impression at Schalke and Nastasic says he has got off to an exciting start, while joking about his youthful appearance. After this interview, the pair will meet up for an “Explaining Schalke” segment on the club’s in-house television channel; Nastasic will be giving the lesson and he is confident that they can both contribute to an upset on Wednesday.

“I think everybody thinks it will be an easy game for City,” he says. “But that’s no problem for us, we don’t feel pressure. We have a good opportunity to see where we are and show that we can fight and play good football against one of the best teams in Europe.”