A Magpies premiership star, Mihocek was playing his first match against his former side when he tried to dodge a tackle from ex-teammate Billy Frampton in the second quarter.
He was caught and went down as the pair wrestled for control of the contest, with a troubled Frampton immediately calling for trainers.
Medical staff quickly called for a stretcher and play was held up for almost 10 minutes before Mihocek left the arena.
In a positive sign, the 33-year-old was moving his arms and communicating with medical staff.
All Melbourne players went over to show support for their teammate before he was driven off, and Collingwood players followed suit.
Play restarted with a free-kick to Frampton, paid for holding-the-ball against Mihocek.
A few minutes earlier, Magpies defender Brayden Maynard was ruled out for the rest of the match with a dislocated right shoulder.
Maynard first hurt the shoulder when attempting a tackle on Harrison Petty in the first quarter.
The Pies vice-captain returned for a short period but popped the shoulder out again when he fell in a contest with Kysaiah Pickett.
Collingwood led 6.3 (39) to 5.7 (37) at halftime.
