Ashby came through the ranks of the West Ham academy and signed for Newcastle United in the recently closed transfer window.
The Magpies paid a £3m transfer fee for the right-back to give their first choice, Kieran Trippier, cover and competition in that position.
Halajko stressed that Ashby is a good technical player with a tendency to get forward and pointed out that, unlike some modern full-backs, the youngster is excellent in aerial duels.
He admits that Ashby has yet to learn different attributes of the game but stressed that the Newcastle star has the potential to become a great modern-day full-back in the future.
“You could see Harrison had the potential to develop into a top player”, Halajko told Chronicle Live.
“His athleticism is really impressive and he’s a good technician.
“He’s a modern day attacking full-back that likes to get forward.
“To be a modern day full-back and play at the top level, athleticism is a non-negotiable and he certainly has that.
“Straight away, you can tick that box.
“If you haven’t got that, you’re not going to play at the top level – and he has.
“Technically he is a good player as well and he’s good in the air for a full-back.
“Sometimes, a modern day full-back who’s athletic is maybe lacking a little bit in the air, but he wins his aerial duels as well as his ground duels.
“He’s still got a lot to learn, as any player of that age has, but he has got some potential attributes that at the top end of the game are very desirable.“
Newcastle will take on Bournemouth at the weekend and Ashby will be hoping to make his debut in Magpies’ colours against the Cherries.