Frank de Boer says it would be a "disaster" is Jasper Cillessen were to leave Ajax, but admits it will be difficult for the goalkeeper to turn down a move to Manchester United.
Cillessen is rumoured to be United's number one target if David de Gea leaves Old Trafford to join Real Madrid.
The 26-year-old is well known to United manager Louis van Gaal, who made him his number one during his time as Netherlands coach.
Ajax coach De Boer is now bracing himself for the possibility of losing his goalkeeper.
"For us it would be a disaster if that's going to happen because it's already very difficult to find a good goalkeeper, especially if you have the best goalkeeper in Holland," he told Sky Sports News.
"We want to keep him and he wants to stay, but if Manchester United come with a bag of money it can be difficult.
"We have to wait but the transfer period still has a long way to go."
Cillessen himself added that he was unaware of any concrete interest.
"I have only heard the rumours so I don't worry about it until I hear something from my agent," he said. "If I don't hear something from my agent, I enjoy playing at Ajax.
"I ask my agent, who also is [Manchester United forward] Memphis Depay's, and he says we don't know anything.
"Of course, it's a great club, Manchester United, but I cannot do anything with rumours. It would be a great step."
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