Tomas Kalas is easing his way into the conversation, contemplating potential commitments ahead with the Czech Republic, when passing mention of Liverpool betrays an eagerness to revisit his breakthrough. Can we start with that daunting Premier League bow for Chelsea at Anfield last month? "Please ask," he insists with a smile. "When the boss said I was playing there was no time for nerves, even if it was a big game, a difficult game, up against Luis Suárez, Daniel Sturridge, Coutinho, Raheem Sterling …" The names of those he helped to blunt trip off his tongue, the part he played in taming a title challenge worn as a badge of honour.

The young Czech defender, who turns 21 on Thursday, has been waiting impatiently to come of age. This should have been a watershed season for Kalas at Chelsea. He had been nominated as a player of immense promise by José Mourinho at the manager's inaugural press conference last June, his reputation established by two impressive seasons on loan at Vitesse Arnhem. The centre-half cum right-back would make a favourable impression on the pre-season tour of the Far East, albeit only against the collective might of the Singha All Stars, a Malaysia XI and an Indonesian select lineup. He defended with maturity and confidence, and appeared to have a role to play at Stamford Bridge.

Then came injury, a period in rehabilitation and, once fit, life as a squad player watching Chelsea's defensive solidity and consistency of selection become the team's greatest strength. Last week brought an apology of sorts from the manager that he has been so under-used. Liverpool was a release; his performance an outpouring of frustration. Mourinho had suggested he would play an under-strength side though of the team selected for the eye-catching 2-0 success that effectively handed Manchester City the initiative in the title race it was the one relative unknown who caught the eye.

"It helped having Branislav Ivanovic alongside me," says Kalas. "I'd played with him in my first game for Chelsea, in pre-season at Portsmouth under André Villas-Boas, so it was like a reunion. Maybe it was also easier for me because … no, I don't want to say that. Actually, I wanted to say I'd rather play against attackers who are quicker with more mobility, than against players like a Demba Ba or a Didier Drogba. I'm still learning how to defend against players like that. Learning in the gym. I played with Wilfried Bony at Vitesse but in training I'd stay well away from him. If you went against Wilfried you might hurt yourself. But against Suárez and Sterling I know one of my strengths is speed.

"We did it, though. Not just me. If you are open and play 'football' against a team like them it might have ended 3-3, and the debut wouldn't have been considered a success. But we played strong defence."

That performance will be what sustains Kalas over the summer. There would have been others this term had his progress not been stalled by the fractured fibula sustained as he dangled out a leg to block a Marco van Ginkel shot in training a few days before Chelsea departed for a second summer tour, of the United States. The Dutch midfielder had been a team-mate at Vitesse and would suffer his own serious injury, a cruciate knee ligament damage, two months later. "Marco didn't score, but my leg was broken," says Kalas. "Injuries are never well-timed but this was even worse than bad timing. I was doing everything I could to show I could be in or around the first team, and then I broke my leg. When I was fit, everything was going well: in the cups, the Champions League, the Premier League. There was no chance to put an inexperienced player in the side."

His involvement was limited to a minute at Arsenal in a Capital One Cup tie in October, and another in the Champions League knockout success over Galatasaray in mid-March. "I understood I wouldn't play. In my position we have John Terry and Gary [Cahill], but you have Ivanovic who can play there too. César Azpilicueta on the right and Ashley Cole on the left. David Luiz as well. So I wasn't knocking on the manager's door all the time asking why I wasn't getting a chance. Even so, I wanted to go out on loan in January, but we didn't have enough players so I had to stay and be part of the first-team [squad].

"I learn in training, and I will learn all my life, but I don't think it's enough at this stage. When I was 17 and first came here I was happy just to work with the first team to make up the numbers. But now the level of my ability is higher. I'm just desperate to play games, like I did at Vitesse. I got used to playing there. Over the two years I played 63 games in a row and then, with one injury, a year has gone just like that. For now it's OK. But let's see what happens next year."

Frustration at a lack of senior opportunity is commonplace among the elite's bright young things, particularly after a week dominated by talk of B teams and League Three. Perhaps more pertinently, Chelsea's under-18s claimed the FA Youth Cup last Monday, and their under-21s are in the Premier League final against Manchester United on Wednesday. Theirs is a slick and expensively assembled academy operation, but how many of their graduates will ever grace the first-team?

Kalas had been recruited at considerable expense, signed for £5.2m, as a 17-year-old. He had made one first-team appearance for Sigma Olomouc at the time with Chelsea having been alerted to his talents after scouting the Czech under-17s, most notably during the European Championship in 2010 in Liechtenstein where England – with Jack Butland, Ross Barkley, Connor Wickham and Josh McEachran to the fore – beat Spain in the final.

Kalas, a goalkeeper with Sigma until he was 12, was one of six defenders to be voted into Uefa's squad of the tournament and aspired to emulate Terry, a player he had admired from afar. He still hopes to end up "a legend in one club", but chiselling out regular opportunities represents a considerable challenge. His agent will speak with Mourinho during the summer to ascertain where the defender's immediate future should lie. "We have to discuss what will be best: if I stay, or should I go and play somewhere and come back even more experienced. Am I ready to step up? If I say 'yes' I show myself to have a big head. If I say 'no', it's like I don't have confidence. So somewhere in between. It's up to me, but if I wasn't confident enough to play the games I'd tell the manager: 'Sell me or loan me somewhere else now.' I have to be confident to play."

He had shown his abilities in Holland in a Vitesse side competing on the fringes of the title race. These days they have the air of a feeder club, where Chelsea players are parked to gain experience; Roman Abramovich's friendship with the club's owner, Alexander Chigirinsky, is well documented. There are four on loan at the GelreDome and Kalas was partnered by Patrick van Aanholt and Gael Kakuta. "Going there was the best decision I've made in my career," he says. "There is a feeling among the guys that we're all from Chelsea and therefore have a common connection.

"I grew up there because, for that first six months at Chelsea, it'd been a fairytale. I'd known all the first-team players from the television, but I didn't recognise them in person. I remember one session early on, training with the reserves, and there was this player there who I sort of recognised. I asked Jan Sebek and Milan Lalkovic who he was and they just laughed: 'Are you crazy? That's Paulo Ferreira.' Seeing these guys in the flesh, and playing with them, was completely different to the telly."

He had arrived in the country alone, a wide-eyed teenager with school-level English but already independent and used to a nomadic lifestyle. His mother and father had long since divorced, his time spent with each parent "moving from one place to another". These days he almost considers himself a Londoner, the round trip of sights he must show friends and family from home almost delivered in autopilot. "I point them towards Tower Bridge, Westminster Abbey, Buckingham Palace, St Paul's, the London Eye, Madame Tussaud's, Tower Hill, The Shard ... all these things. I don't need an Underground map any more because I have my bearings, and I'm never lost in this city."

He did not look out of place in Liverpool, either. Next season he hopes to show he truly belongs.

Tomas Kalas supports Right To Play's 5k run in Battersea Park on 16 May. Right To Play, Chelsea FC's global charity partner, uses sport and play to educate and empower over 1m children every week in disadvantaged communities around the world. Sign up to the run and find out more at righttoplay.com/5krun