The Athletics selected Krook's contract from Triple-A Las Vegas on Sunday.
The left-hander re-signed with the A's on a minor-league deal in November and failed to make the Opening Day roster, but he'll get a chance in the majors after posting a 3.72 ERA, 1.41 WHIP and 42:16 K:BB across 29 relief innings for Las Vegas. Krook has eight big-league appearances under his belt since debuting in 2023 and has struggled in that limited sample size, giving up 15 earned runs with an 8:8 K:BB.