Thornton was optioned to Triple-A Syracuse on Saturday.
Thornton made a surprise big-league debut with a spot start Wednesday in Washington, and he dealt with some growing pains, giving up four earned runs on four hits and two walks while striking out three in 4.1 innings. Thornton, a 6-foot-3 lefty, sits 91-92 mph with his two fastballs and threw his 86-mph cutter 50 percent of the time Wednesday. He earned the spot start by logging a 3.16 ERA, 1.19 WHIP and 40:12 K:BB in 37 innings across seven starts split between Double-A and Triple-A.