Ajax manager Erik Ten Hag says it is "unbelievably annoying" that "an administrative error" meant the club failed to register club-record signing Sebastien Haller in their Europa League squad for the knock-out stages.
The 26-year-old joined the Dutch giants from West Ham in a reported £20.2million deal last month.
But the striker is not on the list of squad changes made for the latter stages of the competition, with Ajax saying they are investigating how the oversight has happened.
Speaking at a press conference as reported by Dutch football magazine Voetbal International, Ten Hag said: "It's no laughing matter, of course. This is unbelievably annoying.
"It's an administrative error. He has not been forgotten. It is indeed a thing with computers: turning a check mark on and off. Something went wrong there. He was definitely on the list."
When asked if the error meant Haller would be unable to compete in the Europa League, Ten Hag added: "Of course we will pull out all the stops. It has huge consequences."
Haller ended an underwhelming spell with the Hammers following a £45million move from Eintracht Frankfurt in 2019 and would have hoped to have hit the ground running with Ajax.
A club spokesperson told ESPN NL on Wednesday: "It is indeed true that Haller is not on the list of players of the Europa League.
"We are working with the KNVB and UEFA to find out where things went wrong."
