"He's still going to be with us and we want to get him back as quickly as we can, but unfortunately the rules state that if you replace a player he can't come back for the season so that's a tough call but Charlie was understanding and wants to try and get back to 100 per cent so that's his main goal."
Prattley, 23, recently signed a two-year contract with Auckland after playing three seasons for Tasman and has been called in to replace Faumuina.
A former New Zealand Under-17 and Under-20 representative, Prattley spent two seasons in the Crusaders wider training group and said he was thrilled to get the call-up from the Blues.
"I'm really excited about the opportunity but also pretty nervous joining the team for the first time this morning," Prattley said.
"It's always been a goal of mine to step up to Super Rugby level and was one of the motivating factors in me making the move up to Auckland so I'm really looking forward to the getting stuck in."
Kirwan said he was happy to welcome Prattley into the fold.
"Sam played well for the Blues' development team so we're happy to have him on board. He's one of those raw-boned, tough and rugged players who covers both sides of the scrum so that's a real positive for us."