Shortly after 11 a.m. it started to get crowded behind the cage on Field 6 of the Mets’ spring training complex. New manager Mickey Callaway and pitching coach Dave Eiland watched intently with scouts Tom Tanous and Bryn Alderson in the crowd. Above, on the second-story, covered deck that oversees every field, former manager Terry Collins watched. While the big boys of the lineup — Yoenis Cespedes and Jay Bruce — were on Field 7 facing Jeurys Familia, all the important eyes were focused on Zack Wheeler throwing live batting practice. The righthander who is still trying to make a successful comeback from March 2015 Tommy John surgery is already in an uphill battle to stay out of the bullpen this spring. Tuesday, a Mets front office source confirmed what had been assumed: that their plan right now is to use Wheeler out of the bullpen to start the season. With the signing of lefthander Jason Vargas to a two-year deal last weekend, the Mets have just two spots open for six guys who were major league starters last season. So much is made of all the pitching talent the Mets came into spring training with, but the team isn’t really sure what they have even if all their starters are healthy. Beyond Vargas, Jacob deGrom and Noah Syndergaard, there are no certainties with this rotation — even if somehow they all remain healthy.