Sean Manaea experienced a setback in his bid to return to the A’s rotation this season.

The left-hander felt soreness in his side while pitching for Triple-A Las Vegas on Tuesday, manager Bob Melvin said. He gave up two earned runs on three hits and a walk in six innings.

“He’s got a little side thing going on, so we’re going to shut him down for a few days,” Melvin said. “Hopefully, it’s not too long.”

Manaea, who’s coming off September shoulder surgery, threw 76 pitches two starts ago and was supposed to be bumped to 90 pitches his next start, but he threw just 70 on Tuesday. The tentative plan had been for him to make one more start before joining the A’s, but his rehab is on hold for now.

Before Tuesday’s start, Melvin had boasted of Manaea’s velocity and command. It’s the second time in his rehab Manaea has been shut down with a side issue. It happened in late June as well, and he recovered quickly, which is why Melvin seems optimistic the latest issue isn’t serious.

“Same thing that happened when we had to shut him down last time,” Melvin said. “He just has some soreness in his right side. ... He had it once before, and it wasn’t a long time he got shut down. Hopefully, it’s just a few days.”

Patterson update: It turns out the A’s didn’t sign Nathan Patterson simply because he registered high velocities in a speed-pitch cage.

They said they were impressed with Patterson’s work with Flatground, an app used to help and showcase pitchers who might not otherwise be noticed by scouts.

Patterson, 23, had gone viral for reaching 96 mph in a speed-pitch challenge booth at a Triple-A Nashville game in August and 94 mph at a recent Colorado Rockies game. He played ball in high school but not college. Nevertheless, the A’s signed him and assigned him to a rookie-league affiliate in Arizona.

“We identified Nathan in early January based off a video he posted on Flatground’s twitter,” A’s baseball operations analyst Ben Lowry said. “We were intrigued by the velocity, which was up to 95, and the fact that he possessed a breaking ball and a changeup.”

Lowry said the A’s continued to monitor Patterson’s videos. In April, scouting director Eric Kubota sent scout Anthony Aloisi to see Patterson at a workout in Nashville. Aloisi was impressed enough to suggest Patterson join a men’s league to work on his craft.

“Anthony saw him and recommended that we sign him,” Lowry said.

John Shea is The San Francisco Chronicle’s national baseball writer.