David Fernandez faces 10 months out of football after a scan confirmed that he suffered a cruciate ligament injury against Celtic on Sunday.
David Fernandez (left) had been tormenting Celtic's defence
The 30-year-old Spaniard, stretchered off after a Gary Caldwell tackle, had been in fine form after joining Kilmarnock from Dundee United.
"I knew right away what happened," the forward said after the 2-1 loss.
"I couldn't feel my toe or ankle. It looks like the cruciate has gone. It doesn't look pretty."
Fernandez, who spent three years with the Scottish champions, had been tormenting the Celtic defence until going off shortly before half-time.
Scotland defender Caldwell apologised afterwards.
It's a big blow as he's just settled in
"It was just really unlucky that he caught me on my knee, but he apologised straight away," said Fernandez.
"I don't think it was malicious at all, so I cannot complain."
But manager Jim Jefferies was more guarded in his comments about the tackle.
"It's a bad challenge because the player got booked and David is on crutches," he said.
"It's up to the referee to decide and he decided it wasn't bad enough to send him off.
"It was only bad enough to book him, so you have to go along with that.
"David will need an operation and will get that done as soon as possible.
"It's a big blow as he's just settled in and, in the last two or three games, has done really well for the team.
"He was enjoying his football again and was causing Celtic all sorts of problems."