Though I'm not a Colts fan, I've always liked Andrew Luck. He's not a stereotypical jock QB, and he marches to the beat of his own drum. Kevin Clark posted an old story he did about how Luck would congratulate and even encourage defenders who tackles/sacked him ('Hey, great hit my man!' etc), which wound up confusing most of them.
To me he was an elite QB who was worn down by playing behind a shitty O-line for too long. All those hits had to take their toll. But there was no throw he couldn't make, and for a big guy he had some wheels.
It's a huge blow to the Colts, but I think Frank Reich can probably game plan for Brissett to make them at least compete for the division title. They might not win it, but 8-8 is achievable. Ironically, that'll hamper them in looking for a franchise QB..