NEW YORK -- Shaun Marcum's Mets debut, already pushed back nearly a week, may have to wait a little bit longer.

Marcum experienced neck pain Tuesday while warming for a simulated game in Port St. Lucie, Fla., the Mets announced, and he was unable to continue. The team plans to reassess the right-hander's status on Wednesday.

Marcum opened the season on the disabled list, with the Mets originally expecting him to return in time for Sunday's game against the Marlins. That may no longer be possible. If Marcum cannot go, left-hander Aaron Laffey is the leading candidate to take his place, with right-hander Collin McHugh also a consideration.

The exact nature of Marcum's injury has been somewhat shrouded in mystery. Marcum skipped a start in mid-March with what the team termed a right shoulder impingement, and he received a cortisone shot in New York. Less than a week later, he cut short a routine bullpen session because of neck pain.

But when the Mets officially placed Marcum on the disabled list Monday, they termed his injury a right biceps strain.

"This is something that he's had for a while," manager Terry Collins said when asked about the discrepancy. "You've got to name an injury. Whatever it is, you've got to give it a title."

The Mets originally intended for Marcum to pitch the second game of the season, an assignment that ultimately went to right-hander Matt Harvey. Marcum, who has a long history of right elbow and shoulder problems, joined the Mets in January on a $4 million deal plus incentives.

The Mets began stretching out Laffey as a starting pitcher late in March, viewing him as rotation insurance should another starter suffer injury.