Derby boss Rooney, 35, is understood to have tackled midfielder Jason Knight in a practice game, and the fallout leaves his first-team star now facing up to 12 weeks on the treatment table.
The Telegraph have claimed that the challenge from Rooney was a '50-50' and simply an accident, but the bizarre situation is especially damaging in light of the club's woes heading into the new season.
Knight picked up the setback during Derby's pre-season camp at Pennyhill Park, a luxury hotel and spa in Surrey, with Rooney having taken part in the drill himself due to his lack of players.
In what has been a testing summer for the former Manchester United man, Rooney has just nine registered senior players on his books, and Knight's absence has plunged them further into chaos.Insurance Loans Mortgage Attorney Credit Lawyer
On Tuesday, Sportsmail reported that the EFL eased the club's transfer embargo, allowing them to sign free agents on one-year contracts or take players on loan for six months if they have fewer than 23 players 'of a professional standing'.
Wayne Rooney faces more scrutiny at Derby County after it emerged he injured Jason Knight
Midfielder Knight (pictured) faces up to 12 weeks out following a tackle from Rooney in training
The incident between Rooney and Knight (pictured last season) has rocked the crisis-hit club
This has become even more of a necessity with Knight ruled out for a sustained period, and comes after Rooney made an open appeal to the club's board at the weekend to add to his threadbare squad.
'It's other people, which I can't control, who need to make bigger decisions,' Rooney said. 'All I can do is try to bring some dignity and pride back to this club. It is clear that has gone in the last few years and it is my job to bring that back.
'I need other people to do their job. It's not ideal, it has been very difficult not knowing what my squad will be in two weeks' time when the league starts. However we can, we need to get that sorted.Insurance Loans Mortgage Attorney Credit Lawyer
'If I am being honest, it would be easy for a manager in my situation to walk away. It's a challenge for me, I'm a fighter. I am grateful to Derby for giving me