NEW YORK -- One of the last healthy players standing from the Mets' Opening Day lineup, Neil Walker has officially joined the ranks of the seriously injured. Walker continues to deal with back stiffness that Mets manager Terry Collins said Tuesday is symptomatic of a condition that could affect him for at least for the rest of this season.
"There's a lot of concern," Collins said.
Though Collins was unwilling to disclose the exact nature of Walker's injury, it is an issue that has caused him discomfort for weeks. Walker was away from the team for four days earlier this month for the birth of his child, starting just two games upon his return before the discomfort resurfaced.
For now, Walker's injury remains a bit of a mystery. Collins said it's possible Walker could be in game shape as soon as Wednesday. It's also possible, Collins added, that the second baseman could be done for the season.
"It's going to certainly be something that he's going to have to manage as he continues to play," Collins said. "If he gets back in the lineup, is he going to be an everyday guy? Yeah, but with days of rest mixed in. I think we'll know more tomorrow how he's going to handle it. But will he play five, six days in a row? No."
When healthy, Walker has been one of the Mets' best players this season, parlaying lengthy hot streaks in April and August into a .282 batting average with 23 homers and 55 RBIs. The Mets say Walker went for testing earlier this week, and claim they will know more about his status when he reports to the ballpark Wednesday.

Though Walker was at Citi Field on Tuesday afternoon, he did not make himself available to media members until after the game, declining comment at that time.
"There's nothing I can really say right now," Walker said. "We'll see where we're at tomorrow. That's all I can really say."
At the least, Walker joins a litany of Mets players dealing with injuries as the club limps down the stretch. The team also admitted Tuesday that starting pitcher Steven Matz could be done for the year with an impingement in his shoulder. Matt Harvey, Zack Wheeler, Jonathon Niese, David Wright, Lucas Duda and Juan Lagares are already finished due to injuries, while Walker, Asdrúbal Cabrera, Yoenis Céspedes and Justin Ruggiano are dealing with medical issues as well.
"The only thing that is helping us is that September 1 is near, so we can get some guys here that will allow those guys to really have some rest without forcing the issue," Collins said. "I understand where Neil's at in his situation. He thinks he's a lot better today, which is why tomorrow we'll know a lot more."