Spoelstra sucked in 2011 for Miami in the first year of the Big Three. They underachieved in the regular season, flatlined and died in the playoffs against Dallas.
Spoelstra couldn't coach the team properly. He kept trying to coach the squad like he had learned as a youngster watching SVG and P Riley coach those Alonzo & Shaq teams where Miami played power basketball. Spoelstra had no clue how to coach uptempo basketball. No clue how to enable LeBron or Wade. The team fell apart in the Finals.
Yet, the team backed him. They saw him as young and that he would learn and develop. And he did. Quickly. He changed from the power basketball to the pace and space system. Heck, he still didn't coach the system well but at least he was coaching the right system. It wasn't until after LeBron left that Spoelstra became a high level coach in this system because he was developing and learning all the time.
Now, Spoelstra had some advantages over Mazzula. The main one being he had 2yrs experience before 2011 when he had established himself as one of the best young coaches in the league when coaching a defensive minded physical unit. He struggled to adapt to a new style but he clearly had ability. They gave him extra time to show he could adapt. And he did.
I would not be surprised if PBS kept Mazzula because he knows him well, likes the way he does things, feels he has potential to grow, and is willing to bet on that growth. Maybe give him a half season next year to show he has learned and grown from this year's failures.
I do not think Mazzula is a shoo-in to be fired. It's more 60-40 in my mind (fired vs not fired).