The wide receiver is yet to score a touchdown in 2013, taking 27 receptions for 470 yards from the first seven games of the season.
The head coach told the team's website: "He's got to improve. He's got to get better. He's got to get to a point where the reliability factor is there as strongly as it always has been.
"Has it been there up to this point? No, but we're saying let's work. Let's get back to work and get this done. We count on this guy.
"The competitor that he is, he's very much aware. Quite frankly, he's always gobbled those balls up. Explanation? I wouldn't offer an excuse, and I'm sure he wouldn't, either."
Nicks is in the final year of his contract with the Giants.