Vitesse are set to be without Theo Janssen for the rest of the season after the midfielder tore his anterior cruciate ligament.
Janssen, 32, sustained the injury during Vitesse's 2-1 home defeat to Feyenoord on Sunday.
The former Ajax man underwent exploratory surgery on Monday, which was expected to confirm damage to his meniscus.
However, Janssen now faces further surgery to replace the torn ligament and is expected to spend at least six months out of action as a result.
"I did not expect this," he told the club's official website.
"Last night (Monday) I went into surgery with the idea that the meniscus was broken. All the tests showed this.
"When I came out of the anesthesia it proved to be a bit worse. Nevertheless there is nothing I can do.
"I was doing good, the team is doing good and we play the kind of football that I like best."
Manager Peter Bosz bemoaned the injury but is hopeful that Vitesse - who sit seventh in the Eredivisie after nine games - can continue to challenge the upper echelons of the division without Janssen.
"Awful. Especially for Theo himself," Bosz said.
"That's the first thing on my mind. At the age of 32 to suffer such a heavy injury is terrible.
"Theo played well and was important for the team. We will have to cope with what we have got."