Starting Poeltl is mostly an issue because of what it does to the bench.
As Is:
Murray, Forbes, DDR, LMA, Jakob
Mills, White, Carroll, Gay
Should Be:
Murray, Forbes, DDR, Carroll, LMA
Mills, Belinelli, Carroll, Gay, Jakob
Starting Poeltl means one wing gets squeezed out of the rotation, probably Beli, who's shooting is desperately needed. Without him in the rotation, I doubt they even attempt 23 3pt per game which is even worse than their attempts per game last season- where they ranked last.
Carroll playing behind DDR means he'll play only around 16 mpg, as opposed to starting or playing behind Gay which would be around 22 mpg. That limits his value to the team. Starting him means the Spurs could space with Murray, Forbes and Carroll and have LMA come out and screen for DDR.
DDR is an excellent setting up Aldridge in pick and pop situations and Aldridge could get wide open 3's or DDR could have an advantage with Aldridge's man being the only line of defense between him and the rim. That could be a lethal attack with 3 other shooters around him.