Leeds United boss Marcelo Bielsa is renowned for being one of modern football's hardest taskmasters when it comes to the fitness of his players.
The Argentine is also a coach who rewards hard work, particularly that of youngsters, which has been reflected in his decision to hand eight teenagers their Premier League debuts already this season.
Latest to make his league bow for the Whites was former Celtic defender Leo Hjelde.
The Nottingham-born, Norway youth international came off the bench midway through the first half in Leeds United's 3-2 win over West Ham last Sunday.
His league introduction came exactly a week after the 18-year-old had made his competitive debut for the club from the start in the side's 2-0 FA Cup Third Round defeat to the same opposition, at the same ground.
Nominally a centre-half, Hjelde replaced Junior Firpo at left-back on his second appearance for the Whites. It is a role he has played previously, including this season for the U23s, but pitched him against in-form Hammers' attacker Jarrod Bowen.
Hjelde received plaudits for his performance in defence, seeing out the game in his secondary position on that flank.
His promotion to first-team affairs reflects stellar work, initially by the scouting department to identify him, but also Hjelde's own efforts to become Bielsa-ready, since signing in the summer.
Speaking to Norwegian outlet NRK after becoming the Scandinavian nation's youngest-ever Premier League debutant, Hjelde said: "It is a bit surreal to get the debut at such a young age, it makes it almost even better."
"Everything happened very fast. There were two players [Firpo and Forshaw] who went down and then I was told to go out and warm up. When I realised I was going in, my heart was pounding.
"The start has been very nice, but it took me a couple of weeks to get into everyday training," he added, on the transition from top flight football in Scotland to the Leeds regime.
"I was told that I had to lose a few kilos to become a Premier League player. You have weight goals, so I was told to lose a few kilos to be at the combat weight they wanted. It was a bit heavy to step down on the food.
Hjelde is son of former Nottingham Forest defender Jon Olav, of the same surname.
Despite being born in the Midlands, Hjelde represents the country of his father, turning out for Norway's U19 and U21 sides earlier this season.
The teenager spent his first four years in England, before moving to Norway following his father's retirement from playing.
There, he came through with local side Trygg/Lade, before being scouted by Rosenborg and then moving on to Celtic.
"In Celtic I was told that I was too light. I ate a little more, and the workouts were nowhere near as hard. It was a shock to come from Scotland. It is a completely different level here. I have received good backing from the club and the players around.
"He [Bielsa] has helped me a lot to achieve my goals. I have been with the A-team since the first day. It has done a lot for me to work under one of the world's best managers. He has been tough at times," the 18-year-old continues.
"There is a lot of running. For my part, there have been brutal training days at times.
"It was probably a 'murderball' session," he answers, describing what he has found most difficult at Leeds. "I remember I played against Patrick Bamford and was tired after two minutes. And then there was half an hour left."
Hjelde will hope to make further first-team appearances between now and the end of the season, but at the very least has given a firm account of himself during his competitive and league debuts.
U23s boss Mark Jackson congratulated Hjelde and teammate Lewis Bate on their Premier League bows after the victory in east London.
"I messaged them both after the game, like I do with all players when there's a milestone in their career and making a Premier League debut is a milestone.
"Like I always say [to them]: 'that's one thing ticked off for you, now, what's next? You've got to go and grab the next thing,'" Jackson told LeedsLive following his side's 2-0 win over Derby County U23s on Monday night.
Hjelde is contracted to Leeds United until 2025.
