Lilly headed in to break the deadlock after half-time before striker Jordan Davies scored from the penalty spot to secure a deserved three points against the 10-man visitors.
The defender, 22 put in a composed and powerful display that drew praise from his boss, who saw his side win back-to-back National League North games for the first time this season.
“With his attributes he should be able to play on the outside of a three because he steps in, he likes to play, he’s mobile and athletic,” Cowan said of Zilly.
“It’s something we decided to do (go to a back four) and he’s revelling in it. If Zak wants to be stronger than someone he will, if he wants to jump higher than someone he will, if he wants to run quicker than someone he will, and he can play.
“It’s consistency. I didn’t think the Gloucester game was a 4-1 game and I thought he was excellent in that game, that’s four games now, and my thing with him is he can’t have a game off.
“I’ve got really experienced players wanting his shirt, the only thing that will concern with Zak is consistency. I’m the one who stuck my neck out to put him back there, I knew he was a centre-back from day one and he’s proving it but he’s got to do it on a consistent basis at this level. And to kick on, I believe he can play at a much higher level, but that’s what he needs to do.”
The Bucks, who remain fourth but just three points off second, welcome 15th-placed Hereford tomorrow evening.
Striker Davies, who joined after impressing on trial in the summer, caught the eye with another busy display and Cowan was chuffed to see the Wrexham-born frontman get off the mark in Telford white.
“Absolutely I was pleased,” the boss added. “I’m ecstatic for him, it’s no less than he deserves.
“There’s a fire in his belly, he plays with no fear, when you see people like that, you’re desperate for them to succeed. I’d love him to continue to do it and he’ll play a part for us this year.”
Cowan confirmed Telford have extended the loan deal of exciting young left-back Eddy Jones from Stoke City from the initial one-month deal until the end of January.
The boss said of Jones, who also impressed against the Lions: “We’re very lucky, I’ve got some great relationships with people and we’re really appreciative to Stoke.
“For them to support us in the way they are is a credit to them, going out their way to help us and help the lad. What a place it is for a young lad to develop. He’s really pleased to be here and is keen to stay even longer.”