Thome has not played since leaving Saturday night's game with tightness on the left side of his lower back. The move was made retroactive to Sunday.
The 41-year-old Thome is hitting .111 with no homers or RBIs and 10 strikeouts in 18 at-bats.
Kratz, who hit .294 with three homers and nine RBIs at Lehigh Valley, will fill Thome's primary role as a pinch-hitter. The 31-year-old Kratz hit a combined .150 in brief appearances with Pittsburgh in 2010 and the Phillies last season.
