"The wide receiver position for a rookie is probably one of the most difficult to learn, so we all felt it was important for him not to miss training camp," Don Yee, Manningham's agent, said in a telephone interview with The Associated Press.
Yee refused to discuss the value of the contract for the speedy receiver, who caught 137 passes for 2,310 yards and 27 touchdowns at Michigan.
Those statistics should have gotten Manningham drafted earlier, but he slipped to the third round because of off-field problems.
Before the draft, he wrote a letter to NFL executives admitting he failed two drug tests while in college. About a year ago, police found a prescription pain killer and a controlled substance in a car in which he was riding with two other men.
Manningham was not charged, but he was suspended for a game.
General manager Jerry Reese said the Giants are confident that Manningham's problems are behind him.
Copyright 2008 by The Associated Press