U's midfielder Alan Judge will be out of action for the remainder of the season.
The Irish international went down in the U's area in the build-up to the Sutton United penalty, and had to leave the pitch on a stretcher before the hosts could take the spot-kick.
He was able to return to north Essex on crutches following the game, but the resultant MRI scan confirmed everyone's worst fears.
Physio Hayden Clifton told the U's official website: "Unfortunately, Judgy has suffered a bad one."
"The injury is a triple rupture in his left knee, with the ACL, (Anterior Cruciate Ligament), PCL, (Posterior Cruciate Ligament), and MCL, (Medial Collateral Ligament), all affected.
"It looked a serious one straight away, and we're looking at it being around a year to a full return to playing for Alan."
Everyone at Colchester United wishes Alan the best of luck for his recovery.