Although the team acquired center options, Kevin Garnett is still the team's go-to option at the center spot, tweeted Paul Flannery of WEEI:
Doc says KG is staying at the five.
— Paul Flannery (@Pflanns) September 25, 2012
Garnett hasn't always been a center, switching to the pivot spot about halfway through last season as the team moved to a smaller lineup with Brandon Bass at power forward rather than pairing Garnett with Jermaine O'Neal. The 6'11 Garnett isn't exactly tiny for a center, and he proved capable of guarding opposing fives, but it's not the position he's played for most of his time in the NBA.
The Celtics acquired options at the center spot in the offseason, signing Darko Milicic and Jason Collins, and also drafted Fab Melo out of Syracuse, but nobody ever assumed these players would see starting roles in Boston this season. Perhaps they'll see increased minutes if Boston opts to go bigger against teams with size -- Andrew Bynum, newly in the Atlantic Division, comes to mind -- but for the most part, they'll play backup roles with Garnett taking the lion's share of minutes at the five.