Yes.
5 days rest and not playing the 2nd game of a back to back against physical Denver (which loves to bang on Manu and he's already had a bruising night before) at grueling Denver with the higher al ude, after having gone into OT the previous night.
So its not as trivial as you make it seem, 1 day extra rest because you neglected to take into account that extra day of rest means avoiding a game where playing in a weakened state, might have elevated consequence. When you play hurt and fatigued, you are more prone to injury.
They don't dig deeper into their reserves before replenishing.
Scale this 1-10. The Spurs after the game against GS is at a 6. They play Denver with their starters... Now they are at a 4.... with 4 days To Rest before they play an Even more physical, and focused, hot, not to mention defending NBA champions, at their home. After 4 days rest, the Spurs are only recovered back to a 6.
Scale this 1-10. The Spurs after the game against GS is at a 6. Now since pop Rested his Stars, After 5 days rest.. the Spurs are recovered to a 9. They will play Boston as healthy as they can be at this point in the season.
(added bonus in resting your Core, the back ups get extended minutes and experience individually and playing as a unit.... AND you sorta mind with the Nuggets, trotting out that line up and still making it a close game..)
So yes that 1 day rest makes a dramatic difference. Even if they lose to Boston it was a beneficial decision by Pop. They play Boston closer to full strength then they would have had the big 3 played tonight, with really no consequence because Denver would have more than likely won this game anyway, and the Spurs would be even more gassed with Boston coming up, and you back ups didnt get any playing time.