Derby captain Richard Keogh could be out until December 2020 after damaging two knee ligaments in last week's car crash.
Scans have confirmed the 33-year-old damaged his anterior cruciate ligament and his medial collateral ligament in the collision that saw two of his team-mates charged with drink-driving.
Keogh, who is into the final 18 months of his Rams contract, could therefore be sidelined for as long as 15 months.
Rams boss Phillip Cocu admitted Keogh was "extremely down" and felt responsible for what happened, before he missed the 3-2 win against Birmingham on Saturday, for which Curtis Davies was made skipper.
The Derby squad were supposed to have been playing foot-golf last Tuesday but it was rained off, and instead they went bowling, followed by a "scheduled team-building dinner".
However, just before midnight, there was a car accident on the A6 in Allestree and Mason Bennett and Wales international Tom Lawrence were arrested and later charged with drink driving.
The Rams said last week that the players involved would "pay a heavy price" for what had happened, but Davies called for the club not to abandon Bennett and Lawrence in the wake of the incident.
Bennett and Lawrence trained with their Derby team-mates on Monday and are in contention to face Barnsley on Wednesday.
There will be a rigorous club investigation but Sky Sports News understands that any punishments from the Rams will be enforced once legal proceedings are complete.
Bennett and Lawrence will appear in court on October 15.