Derby manager Wayne Rooney was responsible for Jason Knight's injury which has made the Ireland international unavailable for 12 weeks, according to The Telegraph.
The crisis-hit Rams only have nine players registered for the 2021/22 season and thus their manager joined the training game himself to make up numbers.
It didn't go well for Knight against whom Rooney went in for a challenge.
"Jason will be out for eight to 12 weeks," the former Manchester United striker said following his side's 2-1 friendly loss to Salford City on Saturday.
"He has rocked his ankle, thankfully, he doesn't need surgery but needs the time to make sure he gets his ankle strong again.
"Of course (it is a huge blow), everyone knows what Knighty brings to us, he is a fantastic player, great energy, and he will be a loss, but we have to deal with that."
