This is a red herring.
He was calling timeouts in the first three games.
It's just y'all focused on the moments where the Heat were capitalizing on runs and putting it down to the fact Mazzulla wasn't calling timeouts as the reason why.
Case in point - Game 1 when the Heat came back in the 3rd quarter; Mazzulla called a timeout when the Heat cut a 9-point lead into a tie game, then Miami came out of that timeout and went on another run (12-6) until there was a next timeout. The timeout made no cotdamn difference - the Heat were just on fire and couldn't be stopped. Miami took a 12-point lead heading into the 4th because they had an outlier of play [46 points in a quarter].
Conversely, Spo has let the Celtics go on runs without calling timeouts over these last three games. Hell, he sat Jimmy for the last nine minutes of the 4th quarter in Game 5, taking away the opportunity for him to not only bridge back the margin, but give him a chance to find some rhythm, form and quality shot generation heading into Game 6. He then came into this game struggling because he couldn't work out what to do once the Celtics stopped falling for his pump-fakes.