The NBA fukked up with the schedule for both OKC and Denver. There was only a 36-hour turnaround between Game 3 and Game 4 (other teams had a lot more rest between games). It should have been amended as it affected the quality of the game.
That is the NBA's fault.
What's not the NBA's fault is, how Denver has failed to acquire proper depth this season and firing their coach late in the piece too (which hasn't helped with rotations). They're playing their starters 40+ minutes every game, coming off 7 games against the Clips in the 1st round, right into the same situation against OKC. To make matters worse for them, OKC and the Clips were #1 and #2 in defense this season.
Their schedule has really been brutal - I saw a stat that if this series goes to a Game 7, Denver would've played 14 games in 28 days.