The Rangers selected Rocker's contract from Triple-A Round Rock on Thursday.
Rocker has dominated at the Double-A and Triple-A levels since returning from Tommy John surgery, logging a minuscule 0.91 ERA and 0.57 WHIP alongside a 47:4 K:BB across 29.2 total innings. He has yet to last more than five frames in any start he's made this year, though he still projects to be a valuable fantasy asset. He will be making his major-league debut Thursday against a Mariners offense that owns the sixth-worst OPS in the majors and has struck out more than any other team.