PORT ST. LUCIE, Fla. -- Two-time Cy Young Award winner Johan Santana, who is slated to open the season on the disabled list, may not make a quick return to the New York Mets.

Santana continues to be plagued by shoulder weakness and is only throwing on flat ground at 90 feet.

He roughly would need to double that distance on flat ground before returning to a mound for the first time since an ill-fated March 6 attempt.

Assuming everything feels OK at that point, he still would need to go through a full spring training-like ramping up.

"I've just got to stay here and work out and get ready," Santana said Saturday morning. "... I'm making progress. It's just I don't know when I'm going to be pitching again. That's the thing: We cannot think ahead. The way we're approaching everything is every day make sure we have a good day."

Left-hander Jonathon Niese will get his first Opening Day assignment for the Mets in Santana's absence. Right-hander Jeremy Hefner will fill the rotation void.

Santana, who turned 34 earlier this month, is in the final guaranteed season of a six-year, $137.5 million with the club.

He underwent surgery on Sept. 2, 2010, to repair a torn anterior capsule in his left shoulder -- a rare procedure in baseball -- and finally returned to the majors last season. Although he had first-half success, including the first no-hitter in franchise history on June 1, Santana faded badly before landing on the disabled list in August.

The Mets attributed the swoon and shortened 2012 season to multiple factors: arduous rehab work the previous winter that left him worn down, an ankle injury caused when Reed Johnson stepped on Santana's ankle during a bang-bang play while the southpaw covered first base, and a lower-back injury.

But there now is an open question about whether Santana's shoulder and his career ever will be the same.

Even Santana does not seem to know that answer.

"I've been in this game for a while," Santana said. "I went through that [surgery] a couple of years ago and I'm still here. So I'm going to battle and try to come back and help as much as I can. When that is going to happen, I don't really know. All I know is I have to work and make sure that I get back into pitching."

Team doctor David Altchek, who performed the surgery, is satisfied the shoulder is structurally sound, Santana said.

"When I talked to him, he said everything was fine," Santana said. "It was just about the strength, getting it back."

Santana described his current activity as a "slow process."

"I'm just building up my strength and throwing more volume," he said. "And then, after that, hopefully I'll be able to start getting on a mound soon. ... With injuries you never know. I got to spring training feeling good. And then, once I started getting to pitch and stuff and I got on the mound, I didn't feel I was making progress."