It was revealed on Tuesday afternoon that the Roosters had signed the 24-year-old on a two-year-deal starting in 2017.
Maguire said he would have liked to have kept Keary in Redfern beyond 2016 but conceded salary cap pressures had played their part in the decision.
"All those great players that you have let go of at some stage; it's about the salary cap and what goes on in club land," he said.
"Luke and I have talked about him staying but also the opportunities for him to leave, and for various reasons he's chosen those.