Harrison, 27, played catch on Tuesday but did not throw his regular between-starts bullpen. He told reporters the pain extends down into his left (push-off) leg, which could explain why the sinkerballer was hammered for five runs and two homers in five innings on Saturday.
Texas' opening day starter has made at least 30 starts and thrown at least 185 innings in each of the last two seasons. Harrison went 18-11 with a 3.29 ERA last summer, earning him a five-year, $55 million extension over the winter. This will be his third career DL stint following shoulder trouble in 2009 and 2010.
Grimm, 24, went 1-1 with a 9.00 ERA in two starts and three relief appearances for the Rangers last year, his big-league debut. He owns a 4.29 ERA in nine starts and one relief appearance at the Triple-A level in his career.
UPDATE: Harrison has an inflamed nerve in his lower back, according to Anthony Andro of FoxSportsSouthwest.com. The southpaw received an epidural over the weekend.