CHICAGO — Zack Wheeler has been throwing bullpen sessions at the Mets’ complex in Port St. Lucie, according to multiple sources. They have yet to set a firm schedule, but the righthander’s progress signals that a minor-league rehab assignment could be approaching.

He has thrown off the mound three times, logging just under 30 pitches in his last session on Sunday. It’s an encouraging sign for Wheeler, whose comeback from Tommy John surgery has been derailed by setbacks. He likely will need a full month-long rehab slate. If he begins soon, he could rejoin the rotation by the middle of August.

Harvey has surgery

Matt Harvey underwent season-ending surgery Monday to treat thoracic outlet syndrome, Terry Collins said.

Harvey, 27, is expected to be fully healthy before the start of spring training next year. He went 4-10 with a 4.86 ERA — his worst season as a major-leaguer — before being diagnosed with the condition.

Surgery was performed by Dr. Robert Thompson, the St. Louis-based specialist who recently performed a similar procedure on Twins righthander and ex-Yankee Phil Hughes.

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People with thoracic outlet syndrome often feel numbness or tingling in their fingers, which in Harvey’s case stemmed from compression of the nerves in his right shoulder. Surgery involves the removal of a rib to alleviate pressure on the nerves.

With Harvey down, righty Logan Verrett has moved into the starting rotation, though the Mets likely will be active in the trade market for other options.

Syndergaard on track

Noah Syndergaard is set to start against the Cubs on Tuesday night, his first outing since July 8, when the velocity on his fastball suddenly disappeared. Collins said he’s encouraged by what he’s seen. “The last couple of days, they say he feels fine,” he said. “I think the rest has been good for him. [Pitching coach Dan Warthen] said the ball came out of his hand great. I’m not concerned.”

Comeback trail

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Reliever Jim Henderson might be closing in on a return to the Mets, though Collins said his fastball velocity still is at 91-93 mph. He has been on the DL retroactive to June 19 with right biceps tendinitis. He has a 10.80 ERA in seven outings but hasn’t allowed a run in his last four appearances, the last two coming with Triple-A Las Vegas.