San Diego also reinstated reliever Matt Strahm from the 10-day disabled list before its game against Washington.
Mitchell, 27, is 0-3 with a 6.47 ERA in seven starts this year. He has walked 26 and struck out 16 in 32 innings.
Mitchell lasted only 2⅓ innings Saturday against the Los Angeles Dodgers in Monterrey, Mexico, in his shortest start of the season.
Jordan Lyles is scheduled to start in Mitchell's spot Thursday against St. Louis.
But Mitchell has struggled from the start.
"The biggest one to me is the walks," manager Andy Green said. "It's hard to pitch in this league walking batters at the level he was for us. That walk rate hasn't been a major issue throughout his career the way it has this year, so we're hoping we can erase that."
Green said Mitchell will have the chance to regain his spot in the rotation.
"Sure. That's all dictated by how well he throws in the bullpen," Green said. "If he goes out there and attacks it and dominates it, we'll definitely look for an opportunity to get him back into the rotation. But he's going to have to show us something. That's being conveyed to him very clearly, with the hope and expectation that he will rise up and show us something."
Makita signed as a free agent in January after spending seven seasons with the Saitama Seibu Lions in Japan's Pacific League. He was 0-0 with a 6.21 ERA in 14 appearances.
Strahm was acquired in a six-player trade with Kansas City on July 24, 2½ weeks after undergoing surgery for a torn patellar tendon in his left knee. He pitched one inning in relief during spring training and made nine rehab appearances, including two starts, with Double-A San Antonio.