Cameron was involved in a marking contest at centre-half forward as Rohan came across from the side, with his left shoulder accidentally clipping the key forward's head.
Cameron looked in a bad way and stayed down for several minutes while he was assessed by club medical staff, holding up play during the first quarter.
The 30-year-old was conscious when he was taken off the field on a stretcher about eight minutes after the collision, and gave a thumbs-up gesture to the crowd at GMHBA Stadium.
Cats substitute Jack Bowes was activated as Cameron's replacement.
Cameron's injury is a huge blow for the Cats, who led 2.1 (13) to 0.2 (2) when their leading goal-kicker went down.
Rohan was visibly upset after the collision and stayed with his teammate while medical staff assessed Cameron on the field.
Rohan found the ball soon after the match re-started and received warm applause from the Cats crowd.
He then brought them to their feet with a team-lifting goal before quarter-time, helping the Cats to a 3.4 (22) to 2.1 (13) lead at the first change.
Melbourne had their own injury scare during the first term when Jack Viney left the field nursing his right shoulder, having hurt it while tackling Cats defender Tom Stewart.
Viney was checked on the sidelines and the tough midfielder soon returned to the action, attending the next centre bounce.
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