Neil Lennon has told Vasilis Barkas he needs to improve on several fronts after the Greece goalkeeper was dropped for Celtic's 2-0 win over Hamilton.
The 26-year-old, who was signed for £5m in the summer from AEK Athens, has failed to impress and was replaced with Scott Bain against Accies.
Lennon said he did not think Barkas had "settled" since moving to Scotland.
"He's not the presence at the minute that he can be," the Celtic manager told BBC Scotland.
"You will probably see the best of Vasilis as we go along, not just at this moment. I have spoken to him on a number of occasions over the course of the season. We're hoping and expecting more from him as we go along.
"Sometimes you take it for granted that these guys are going to come in and adapt straight away. Some take a bit longer and he is in that category.
"He has admitted on a few occasions that he probably hasn't been at his best. He can improve his physicality, shot stopping and general influence around the box.
"He has good ability, I don't think we have seen the best of him yet. Scott will continue in goal."
Commenting about Bain on his return to the starting XI, Lennon added: "I made the decision to bring Scott back in.
"He has been knocking at the door. I think he has done OK. He was comfortable tonight."