Hey Spurs's fans, what's the deal with Tiago Splitter's playing time?

When the Spurs played the Celtics he lit us up in a very short amount of time with some fantastic physical post play and he kicks ass on pick and rolls. He's okay on defense and rebounding and his free throw shooting has improved big time this year so what gives on why he plays so little? His PER and per 48 minute numbers are amazing so what gives?