The former Aston Villa striker, who appeared to be settling in perfectly to his new surroundings, controlled the ball and played a 20-yard pass, but in so doing he twisted awkwardly and fell to the ground.
It seemed serious from the moment a number of players turned away in shock and so it proved when it was confirmed that the forward had suffered a leg break just above the ankle.
Collymore was immediately rushed to Derby Royal Infirmary, from where the club doctor revealed that it is, hopefully, not career threatening.
The game itself started very brightly with The Foxes nearly taking the lead when, following a Steve Guppy corner, Darren Eadie directed a shot towards goal but Mart Poom saved easily.
Gerry Taggart was a relieved man in the ninth minute when his sliced clearance flew agonisingly wide of his own goal with keeper Pegguy Arphexad stranded.
The Rams` first real attack resulted in a goal for Craig Burley. The Scottish international picked the ball up 25-yards out before firing in a rasping shot that flew into the bottom corner.
Martin O`Neill`s men are nothing if not resilient and Ian Marshall epitomises this and he forced a fine save from Poom after half an hour.
Leicester`s woes were compounded in first-half injury time when they conceded two goals.
Firstly, Rory Delap glanced a fine header past Arphexad, but his task was made easy by a superb cross from Seth Johnson.
Nothing appeared to be going for City and this was proved when, following a superb save by Arphexad from a Horacio Carbonari blockbuster, the ever impressive Delap made the rebound his own before crossing for Dean Sturridge to nod into an unguarded net.
O`Neill refused to admit defeat and he employed Matt Elliott as an auxiliary striker, a move which nearly reaped dividends moments into the second period, when the Scot forced a fine reflex save from Estonian international Poom.
City`s woes were compounded in the 68th minute when Neil Lennon was dismissed for a second bookable offence by referee Graham Poll, but if O`Neill thought things could not get any worse he was wrong.
On 70 minutes, Darren Eadie became the second Fox to leave the field on a stretcher after a hard, but fair, challenge by Delap left him clutching his leg.
With nine minutes remaining Steve Guppy had, what appeared to be, a perfectly legitimate goal disallowed for offside.
Derby controlled the remaining moments to grab three vital points in their battle to move away from the drop zone.
