NEW YORK -- When Zack Wheeler takes the mound on Thursday for Triple-A Las Vegas, he will almost certainly be making his final Minor League start.

General manager Sandy Alderson has all but confirmed June 18 as the date of Wheeler's big league debut, saying on Monday on WFAN radio that "unlike most speculation, this probably has some accuracy."

Alderson did leave himself some wiggle room, presumably in the event of weather or injury issues, but the Mets have long eyed next week's series in Atlanta as a leading possibility for their top pitching prospect's debut; with five games against the Braves in four days, the Mets would have at least needed to use a spot starter on Monday or Tuesday.

In the meantime, seemingly everyone around the club is preparing for Wheeler's arrival. Manager Terry Collins has spent the past two weeks evaluating Dillon Gee and Jeremy Hefner, one of whom may lose his rotation job to make room for Wheeler. In theory, that choice can wait until June 23, when Wheeler's turn in the rotation will come up a second time.

Jay Horwitz, vice president of media relations, spent his off-day last Monday flying from Washington to Fresno, Calif., where he gave Wheeler a face-to-face crash course on media training. Horwitz traveled roughly 6,000 miles on the 24-hour trip.

Wheeler is 4-1 with a 4.14 ERA for Vegas and has struck out 66 batters in 63 innings, but he was roughed up in his most recent outing, giving up four runs on eight hits in 4 2/3 innings. He ranked seventh on MLB.com's 2013 Prospect Watch, and is second on the Mets' list, behind catcher Travis d'Arnaud.

"You hear stuff," Wheeler told reporters in Las Vegas after that start, referencing rumors of his imminent promotion. "You can get excited. There's nothing wrong with that."

The Mets might have called up Wheeler sooner if not for a desire to wait until after the Super Two cutoff had passed. Though it's not a hard deadline, the cutoff determines which players who have between two and three years of service time will be eligible for an extra year of arbitration, thus escalating their salaries and making them eligible for free agency one year earlier.