Yes, head coach does matter.
I would say the coach contributes to about 30% of a team’s success or failure. People think it’s just about the players.. yes in basketball the players matter more compared to say soccer or football or hockey because there’s only 5 players on each team on the court. The less players there are on the floor or the field, the more the individual players matter. However that doesn’t mean the coach doesn’t matter.
We’ve seen that great players have often needed great coaches to elevate their abilities and elevate the team.
The Bulls in 1989 had Michael Jordan, Scottie Pippen, Horace Grant, Bill Cartwright, Craig Hodges, John Paxson. Basically the same core group of guys except with Doug Collins as coach. Now Collins is a very solid coach – but he’s not Phil Jackson. Collins was able to take that Bulls team to the ECF – but he couldn’t take them to the promised land. Jackson took that same roster and molded it into a championship roster.
Same with Shaq and Kobe. 1999 Lakers had incredible talent – prime Shaq, Kobe (yes he was a starter that season!), Glen Rice, Rick Fox, Derek Fisher, Robert Horry. They got swept in the 2nd round. Phil Jackson comes in 2000, takes that exact same roster to the championship.
Warriors in 2014 had Mark Jackson as coach – similar to Doug Collins he was a very solid coach, but he wasn’t an all time great coach like Steve Kerr. The 2014 Warriors roster had Curry, Klay, David Lee, Draymond, Iggy, Harrison Barnes, Andrew Bogut. Lost in the 1st round. Steve Kerr came in the next season, made some adjustments, and they win the championship with the same roster.
2021 Celtics.. the roster had Jayson Tatum, Jaylen Brown, Marcus Smart, Kemba Walker and Brad Stevens as coach. Stevens was a solid coach.. but they lost in the 1st round to the Nets. Next year.. they have Ime Udoka as coach, and the same core stars except replace Kemba with Al Horford, and they not only sweep the Nets but they go all the way to the Finals and became known for their lockdown defense.
So, you can see how a great coach makes all the difference. The reason why it seems that coaches don’t matter in the NBA is because most coaches are either bad, mediocre or just good. There isn’t a big enough difference for most coaches. There’s only a few all time great coaches – Phil Jackson, Poppovich, Steve Kerr, Pat Riley etc to really matter and so most of the time you don’t see too much of a difference when you see coaches getting fired left and right. Most teams when they change coaches – it’s mostly a sideways move rather than an upgrade. When a great coach comes in though, then it matters a lot, as you can see from my examples.