"It's difficult for some of these young guys, they need to make big decisions. You've seen Patrick make a pretty tough decision... he's got a 15-year career ahead of him... but it's a big decision in World Cup year.
"I've been proud of the way they've handled it, really, because it's pretty disappointing... they're young enough to play for us next year and go on and play the [British and Irish] Lions and the [following] World Cup so that's what they need to think about - get themselves right physically and hoe into it next year."
Kirwan, meanwhile, has named a young team to play the Crusaders at Eden Park on Saturday due mainly to injuries but also a decision to play Hamish Northcott and Matt Vaega ahead of former All Blacks Francis Saili and Frank Halai, two players moving on at the end of the season.
Kirwan defended the decision to keep Saili and Halai out of the match-day squad, saying he wanted a closer look at Northcott and Vaega.
When asked whether the experience and quality of Saili and Halai might have made the Blues more competitive against a Crusaders team still with a chance of making the playoffs, Kirwan said: "That's a fair observation, but it's about us more than anyone else.
It's an opportunity for these young guys to step up and take on some of the best in the world who are desperate to win so for us that's a real character moment. For us the season is gone, it's not for the Crusaders, but if we come out of Saturday night and say 'wow those three or four boys are looking really good next year' that's a positive for us.
He added: "Obviously we're taking the field to win.
"It's hard on those guys who are leaving but we have to make some solid decisions for the future."
Fullback Charles Piutau remains on track for next week's match against the Highlanders following his knee injury.
Waitemata club player Airi Hunt, who is yet to play in the ITM Cup, has been named at No8.
Captain James Parsons, who will start at hooker for his 49th match for the Blues, has re-signed with the franchise until 2017.
Blues team to play the Crusaders at Eden park on Saturday, kick-off 7.35pm is: Matt Vaega, Ben Lam, Hamish Northcott, George Moala, Melani Nanai, Ihaia West, Jimmy Cowan, Airi Hunt, Brendon O'Connor, Joe Edwards, Josh Bekhuis, Will Lloyd, Ofa Tu'ungafasi, James Parsons (c), Sam Prattley. Reserves: Keven Mealamu, Nic Mayhew, Sione Mafileo, Hayden Triggs, Chris Vui, Jamison Gibson-Park, Simon Hickey, Matt McGahan.