Gary Harkins will miss Tuesday's League Cup tie against Stenhousemuir after being stretchered off in Kilmarnock's 3-1 win over Dundee United.
The midfielder came off worse in a clash of heads with Gavin Gunning and manager Kenny Shiels has ruled him out.
"He will be out for Tuesday and we've got a really tough game against Stenhousemuir, who are a very good team," Shiels told BBC Scotland.
"That's the second time he has had to come off early this season."
Shiels praised Rory McKenzie for his debut goal and another of his summer signings, Spanish striker Borja, who scored the second from the penalty spot.
The 18-year-old McKenzie pounced for the opening goal shortly after the break after full-back Barry Douglas tried to dribble out of his own penalty area.
"It was opportunistic," said Shiels. "He was in the right place to pounce.
"If he had dropped down and been lazy, he would not have been around to score.
"I think Rory has done very well."
The third goal was scored by another teenager, 19-year-old Jude Winchester.
"When young players score, you do get more satisfaction," said Shiels.
"It integrates the community and the supporters see it as one of theirs.
"We lost Harkins early on, but I thought the young lads who came in did really well.
"And Borja stepped up to the plate and did very well."
Shiels suggested that few teams would score three against a strong United defence this season, the Tannadice side having started the day top of the Scottish Premier League.
"I think it is a really good result for the spirit and confidence of the team," he added.
"I think the weather had a significant part to play. The rain just come on at half-time and it suits the team that plays on the ground and it enhanced our performance because we go it down a bit more.
"It became a little more difficult to defend for Dundee United because of our mobility and movement.
"But they are a really good team and I'm really pleased to beat them. I thought they were outstanding from the first whistle."