Mfuni hadn't even made his senior debut when he signed for the Hornets aged 17 at the end of January, but is now a key defender and has established himself as a sensational talent.
Capable of playing at centre-back or left-back, the conversation is as much about what heights Mfuni could scale in years to come as what he can achieve for Watford between now and May.
Still believes anything is possible for the youngster, who he claims compares to a current first-team City option he has worked with previously.
The head coach said: "He's fantastic.
"He's the third Manchester City defender I've had the chance to work with.
"We had Abdukodir Khusanov at Lens last season and then Juma Bah, who's a CFG (City Football Group) player at Nice currently.
"In terms of potential, he's right up there. That's stating the obvious.
Abdukodir KhusanovAbdukodir Khusanov (Image: Reuters)
"Everybody who was at the Vic on Tuesday saw again what a good player he already is and he's only 18. To be honest, I think his ceiling is whatever he wants it to be, as simple as that.
"If he keeps his feet firmly on the ground – which so far he has done – and keep his focus on the pitch, makes sure his body is in tip-top shape, then he'll decide how far he goes."
Earlier this week, Mfuni revealed which one of the more senior Watford defenders has proved a particular help in transitioning to first-team football.
He told the club site: "James Abankwah is one of the people I speak to a lot to and I’ve learnt a lot from him, because he has been through what I’ve been through.
"It’s good to see things from his point of view.
"The game is completely different to academy football. It’s a great learning experience and I’m taking on board everything from every player and member of staff."