NEW YORK -- Second baseman Luis Castillo left Tuesday's Mets home opener against the Phillies after four innings with right knee soreness. He is day-to-day.

Castillo felt discomfort in his knee while running after a foul pop in the first inning. He remained in the game for three more innings, however, taking two at-bats. Backup infielder Damion Easley replaced Castillo in the field to begin the fifth.

"He just said he was sore," Mets manager Willie Randolph said. "He came to me about the second or third inning and told the trainer he was sore, so I got him out of there."

Castillo attributed the soreness to the cold weather -- game-time temperatures dipped into the 40s -- which he wasn't used to after opening the year in Miami and Atlanta.

He had battled knee ailments constantly before joining the Mets last August, and had offseason surgery to clean out debris from both knees. Still recovering from that surgery heading into Spring Training, Castillo continued to nurse his right knee for weeks after reporting to camp, missing well more than half of the Grapefruit League schedule.

Though Castillo expected to be out of the Mets' lineup on Wednesday night against the Phillies, he wasn't sure how much longer the injury would keep him sidelined.

"I'll play when I'm ready," he said. "I want to be 100 percent before I start playing."

Castillo is batting .182 with one stolen base in six games this season.