Niese was forced from the game, a 7-2 loss to the Los Angeles Dodgers, but is expected to make his next start, Sunday against the Philadelphia Phillies. X-rays were negative, and Niese was diagnosed with a bruise.
Niese will know more Thursday, after attempting his customary between-starts throwing session in the bullpen.
"It felt kind of like a stinger, like it hit a nerve," Niese said. "It obviously was painful. But then it kind of locked up down there. I couldn't move my toes. It was kind of scary at first. But then, coming in here, letting it settle down, it's just a bruise."
Manager Terry Collins said the ball struck Niese between the Achilles tendon and ankle bone.
Niese, the Mets' Opening Day starter, briefly squatted by the first-base line as trainer Ray Ramirez and Collins checked on him. He then deliberately left the field.
"If you're ever going to get hit there, I guess that would be as good a spot as any," Collins said. "Certainly the fear was as soon as he couldn't move his foot ... [it] was a bad sign for us. Obviously, he's fine. He's walking around. He's getting better as the night goes along. So he'll be OK. He shouldn't miss a start.
"When it happened, your first worry is something serious is there and how we're going to fill it. But you always wait until the right people examine him, let the doctors do their job, and there's no reason to panic. What we had to figure out was how we were going to get seven innings of relief pitching."
Niese, by game's end, already had iced the lower leg multiple times.
"It's a lot better than what I thought it was going to be," he said. "It's just a bruise. Initially it hurt pretty bad."