The Jets defense is one of the top units in the NFL, but the defensive back has struggled in pass coverage, allowing a 55-yard touchdown against the Pittsburgh Steelers last week.
"A 'C,'" Cromartie told reporters. "I'm blunt honest. I give myself a 'C.' It's been a tough year for me.
"I don't consider this being me playing at a Pro Bowl-caliber level at all. I definitely want to pick my game up and make sure I'm playing to the best of my ability."
The Jets face the New England Patriots on Sunday.