He was getting reps during that stretch. Duncan sat a couple of b2b's and Tiago was getting more minutes and more shots. His post-up attempts dropped dramatically after the all-star break.
His spacing, off-ball movement and passing were well above average, but he really needed to work on adjusting to the athleticism of NBA bigs. The only way he was going to get better in the post was to get more attempts in the post. Outside of a few games here and there, Tiago never really got that opportunity. And it's not like he was hurting the team with his post-up offense; he ended the season with a higher post-up FG% than Duncan.
No one can argue that Tiago's offensive game wasn't incredibly efficient and impactful during the regular season. Overall, his defense was average to above average, but on offense he made the most out of every minute he got. Tiago can certainly be more than a pick-n-roll player, but he's going to need minutes and he's going to need reps. If the Spurs can't figure out a way to play him with Duncan, then they need to find a player who can.