O'Brien (0-1) allowed two runs on two hits and three walks while striking out two over 1.1 innings as he took the loss against the White Sox on Tuesday.
O'Brien's MLB debut was not a smooth one after he started in place of Luis Castillo, who was placed on the family medical emergency list Monday. Both hits he let up went for solo home runs, with Luis Robert going yard in the first and Yoan Moncada repeating the feat to lead off the second. He also scuffled with command, throwing just 22 of 42 pitches for strikes while walking three batters. The 26-year-old has also struggled with giving out free passes at Triple-A this season, so his inability to find the strike zone consistently wasn't all that surprising.