Congrats to Messina. Some thoughts:
1) I've heard he really wanted to be a head coach. But I've also heard he's not the most impressive interviewee. Language barrier probably played a role.
2) Would he have been a success NBA head coach? I don't know. He's well-respected by just about everyone so I'd say he would have had a decent shot. I was never overly impressed during the instances he took over for Pop but that admittedly was a small sample size.
3) After losing Udoka and Messina, I'm almost certain that the Spurs will bring in at least one additional assistant coach from outside. That's what history tells us they will do. There's only so much they can elevate from within.
4) Will Becky become the lead assistant? Maybe but I'd guess not. It's just a bit too soon -- that'd be two big jumps from her current role. Plus, in order to get the highest quality assistant, the Spurs will need to dangle the lead assistant role. I think the chances are higher for Becky to slide into Ime's seat.
5) I think there's a higher chance the Spurs hire two outside assistant coaches than there is they stand pat.
6) If the Spurs do stand pat, that'd tell me the Spurs are planning to hire an outside coach after Pop retires and plan to let him bring in the assistant coaches of his choosing. In that scenario, there's no use spending money on assistant coaches who could only be in San Antonio for a season or two.
7) I still like the idea of bringing back Avery Johnson as an assistant. In retrospect, he did great work with the 2006 Mavs. That wasn't a very talented team and he had them within an inch of a championship. Plus, some of his innovations (isolating Dirk midpost around the free throw line, constant pick-and-rolls to force mismatches, killing teams who employed slow centers) are now used league-wide and can be traced back to his mid-00s Mavs teams. Sure, he's gruff and not super player-friendly, but he's smart, dedicated and not afraid to share his opinions, to say the least.