Lackey, who has been out the entire season after undergoing Tommy John surgery, threw 45 pitches in the bullpen at Yankee Stadium, and Valentine said "35 to 37 of his pitches were excellent."
"He threw with an ease of motion and with perfect location," the manager said. "I was very impressed with what I saw. He's feeling really good about his progress.
"From what I remember the last time I saw him throwing and what I saw today, it looked like different people. He said it was a different effort, he was throwing through so much pain in the past."
As for the next step in Lackey's rehab, Valentine said he's "going to try to spin more balls on flat ground" and that he, assistant pitching coach Randy Niemann and trainer Mike Reinold are thinking about having Lackey get up and down to throw simulated innings.
Lackey was 12-12 with a 6.41 ERA in 28 starts last season, his second with the Red Sox after signing a five-year $82.5 million contract as a free agent.
Valentine also said designated hitter David Ortiz took batting practice on Saturday and will undergo some agility drills and take another round of BP on Sunday as he tries to return from an Achilles injury.
Right fielder Cody Ross was out of the lineup on Sunday after striking out four times on Saturday, but Valentine said, "He has a good pinch hit in him tonight."