Burnett underwent surgery for a broken right orbital bone on March 2 and was placed on the 15-day disabled list retroactive to March 26. He threw a bullpen session on Wednesday at Chase Field in Phoenix.
"We'd like to see him go past 80 pitches and as close to 100 as he can, and be productive and efficient, that would be our goal," Pirates manager Clint Hurdle said.
Burnett made two rehab starts with Class A Bradenton and went 0-2 with a 12.79 ERA. He threw 81 pitches and gave up seven hits and five runs in four innings for Triple-A Indianapolis on Monday.
"The results really don't matter that much," Hurdle said. "You are dealing with a whole different animal down there with A-ball hitters and big league hitters. It sounds funny, but it's real. If you follow these things you see it, so what we're looking for is the crispness to the pitches, the location and is he following the glove, does he have arm slot command and is the ball sinking, what's his slider doing, what's his curveball doing and changeup."