Tonkin (0-1) took the loss Friday as Atlanta fell 5-4 to the Padres, giving up one run on one hit and one walk over two innings while striking out one.
The 33-year-old righty hadn't pitched in the majors since 2017, but he won one of the final spots in the bullpen this spring and has been solid in a low-leverage role so far, posting a 3.38 ERA, 0.56 WHIP and 4:1 K:BB through 5.1 innings over three appearances. Tonkin has no minor-league options remaining, so he would be subject to waivers if Atlanta tried to send him down to Triple-A Gwinnett for a fresh arm.