I would still throw him in, he just i will miss a few tackles. Realistically every halfback does; Kimmorley, Mullens, Sandow, Pearce, Thurston, Prince, Orford.ok..but weren't you the one who wanted to throw him in straight away.
Theya re all crap defenders, and normally have a forward covering for them. Play the kid next to hindy and tell the forwards to look out for him... that is their job.
Sandow averages somthing like 8 miss tackles a game, I would expect something like that from Mortimore at this stage until he physically mature. ATM i think he is mentally there, and will probably learn more being in first grade them Toyota cup. Playing against guys like Prince, Thruston and Lockyer, the kid will learn alot.