Coates insists the Potters have contingency plans in place.
"I feel he's as good as anybody in the Premier League," Coates told The Sentinel.
"I don't know anyone who is better.
"You always have to be realistic about these things. If a call came from a top European club and was attractive to him, then you have to look at it.
"It wouldn't be much of a surprise to me and that's why we have contingency plans in place with other goalkeepers at the club.
"We would be reluctant to sell him because you want to keep your best players, but equally you are realistic about things. You also have to consider the player's view."
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