Richard Foster insists talk of Vasilis Barkas earning a Celtic redemption is way off the mark as he advised the Greek to "remember to put your hands in your gloves".
The goalkeeper arrived from AEK Athens in the summer for a reported £5million but has struggled badly in his first season.
Barkas had one of the worst save percentages in the Premiership before being reinstated against St Johnstone on Wednesday, and had let in four of the last six shots he'd faced.
However, the 26-year-old kept a clean sheet in a comfortable win against the Perth side, including making a smart stop.
One caller on Go Radio insisted that the Greek hasn't been given a proper chance to impress.
But Foster hit back: "It's an important position, of course it is.
"(Karamoko) Dembele coming in, he's young. He's 18, he deserves an opportunity to make mistakes.
"Barkas has signed for £5million, this is not a time to 'give him his opportunity'.
"He should be coming in as a number one goalkeeper. If you're getting signed for £5m you should be commanding, you should be dictating to your defence what you want them to do.
"We haven't seen any of that from Celtic, defensively they've been all over the place at times and that comes from having a goalkeeper who clearly doesn't speak. Or if he does, he doesn't tell the players the right things.
"They've lost so many goals at set plays. All the best goalies I've worked with are usually the loudest guys on the pitch.
"Because they can see everything and they can tell you what they want you to do, and they can tell you where to pick up and where your man is.
"It looked like this season they haven't had that and, like I say, you're getting signed to go to Celtic - not just any old club, one of the biggest teams in Scotland - that's not a time to 'get a chance', you should be in there as an established goalkeeper.
"And you should also remember to put your hands in your gloves..."
