Walters, who was placed on the disabled list with right shoulder inflammation on June 14, struggled in the early innings against Triple-A Scranton/Wilkes Barre.
He gave up two runs in the first inning and threw just one first-pitch strike to the first seven batters he faced. But he settled down and ended up surrendering three runs on five hits over four innings.
"He threw 40 strikes and 30 balls, so he was misfiring," Twins manager Ron Gardenhire said. "He got better as the game went along so he was rusty. So that tells me probably another start. He started locating it better at the end, but he couldn't locate his fastball early."
Gardenhire said there's still a chance Walters might make one more rehab start after Friday, because the Twins are off from July 9-12.
Walters, 27, posted a 5.40 ERA in seven starts before being placed on the DL on June 14 after allowing four runs without recording an out against the Phillies on June 13.