Michael Appleton has warned Ashley Harris he has to up his fitness to cope with the rigours of first-team football.
The teenager impressed after making his full Pompey debut against Derby.
Yet he had to be substituted after 60 minutes for Marko Futacs having been struck down with cramp.
Up until that point it was a tremendously encouraging display from the Academy attacker.
Appleton believes Harris’ forced exit was down to fitness problems adjusting to the pace of the first team.
Considering the youngster’s age and it was his debut, it is a problem the Blues boss can empathise with.
But Appleton is well aware the 18-year-old needs to boost his fitness to get him through first-team occasions in the future.
He said: ‘Ashley obviously needs to get a bit fitter having come off with cramp.
‘It was a big day for Ash and he did okay.
‘The crowd got right behind him from the start and I’m sure he will take that experience on board.
‘But it’s a lot quicker and stronger than it is in youth-team football.
‘I’m sure that will be the case in the early part of his career, maybe next season he has got to get up to fitness.
‘I thought he did well, though. He was very sensible, mature in terms of his positional play and he was good. It was a solid performance from Ash.’
The Purbrook youngster has made four appearances for Pompey so far, three as a substitute.
He has emerged into the first-team set-up during the past two months after starring for the Academy.
Harris has scored eight goals in 21 games for Andy Awford’s side.