Jetta, who has had a tough run with injury, will miss 12-16 weeks with an ankle injury.
He suffered the injury playing for Casey against Box Hill in a VFL practice match on March 10.
Couch, the son of 1989 Brownlow Medallist Paul, gains an opportunity after showing some impressive form in the NAB Cup.
He was drafted in last December’s NAB AFL Rookie Draft.
Rookie James Magner is Melbourne’s nominated rookie for the season. He is eligible for all AFL matches this season.
The Demons also have ruckmen Max Gawn and Jake Spencer on the long-term injury list. Both are sidelined with knee injuries.
At this stage, no other Melbourne rookies have been promoted for Gawn and Spencer.
Michael Evans, Kelvin Lawrence, Daniel Nicholson, Jai Sheahan and Leigh Williams are Melbourne’s other rookies.
