Johan Santana already has the Mets guessing whether his left shoulder will be ready for Opening Day.

After watching Santana throw a lackluster bullpen session on Wednesday, manager Terry Collins yesterday said the left-hander will be scratched from his March 2 spring training start against the Marlins and focus on building arm strength. The new plan calls for Santana to make his exhibition debut somewhere between March 10 and 15, according to general manager Sandy Alderson.

The Mets have ruled out a structural problem with Santana’s arm, but said he needs further conditioning.

“I haven’t gotten on the mound in a while and it takes time to get everything adjusted again and to get the pitching mode again,” said Santana, limited to playing catch over the winter as he rested his shoulder. “It takes time, so that’s all we’re doing. That’s not a setback at all, it’s just that it needs time to build up and to feel good.”

Collins had concerns about Santana’s arm speed last Sunday during a bullpen session, but thought the unusually cold weather — temperatures were in the upper 40s — played a factor. But after watching Santana throw again Wednesday, on a typical tropical Florida day, the manager became concerned.

“He didn’t have the arm speed, you could just tell,” Collins said. “One of the things that last year at this time he really showed was the great feel for the change, even though his fastball command, he was trying to build up some velocity, but at least he had the arm speed, and the other day I didn’t see much arm speed.”