Middlesbrough boss Tony Mowbray has confirmed that Kevin Thomson was knocked unconscious during a 2-0 defeat to Leeds on Sunday.
The unfortunate Scottish midfielder lasted just 10 minutes of his return to the Boro first team before being forced from the field.
He was introduced as a 62nd-minute substitute but soon found himself being carried off on a stretcher after taking a boot to the head.
Thomson came around shortly after the match, but he could be facing up to another spell on the sidelines as he nurses a concussion.
"He was out cold, he got knocked out and wasn't conscious," said Mowbray.
"But he's come around now and seems okay, but we've got to be very, very careful with concussion.
"I think Kevin's just had a dreadful time since he's come here. First he broke his leg on a treadmill at home and then it's been injury after injury after injury.
"He's had an illness for the last fortnight and had just got himself ready to come back into the team but he's off again within 10 minutes.
"I'm sure he'll go through a spell where he'll show everybody what a quality player he is. He's a high quality player and I'm sure at some point in the future he'll show everybody what a talent he is."