SANTA CLARA, Calif. -- Michael Crabtree is confident he can play a big role for the San Francisco 49ers as a rookie even though he's months behind.

Crabtree signed a six-year contract early Wednesday after a drawn-out negotiation process that had some wondering whether the star wide receiver would ever show up this season.

"It's a lot of relief off my shoulders," Crabtree said when formally introduced at team headquarters.

Crabtree gets a six-year contract that can turn into a five-year deal if he lives up to his potential as the 10th pick in the 2009 draft, a source told ESPN.com's John Clayton.

According to the source, Crabtree has a couple of scenarios that could void the sixth year of his contract. If he goes to a Pro Bowl during his first four years, along with playing 80 percent of the offensive snaps during a separate season in which the 49ers reach the playoffs, the source said the contract voids to five years. Another provision would be going to a Pro Bowl in the first four years along with going to a second Pro Bowl in five years.

According to the source, the Crabtree contract is for six years and $32 million that could maximize to $40 million if he achieves every incentive in the deal. The guarantee in the contract is $17 million, but he is expected to make $19 million over the first three years and $23 million over the first four years. The five-year deal is worth $28 million, according to the source.

Crabtree headed out Wednesday for his first practice as the 49ers prepare for Sunday's home game against Atlanta.

"Everybody came to a reachable agreement and it happened," Crabtree said, sporting a red No. 15 practice jersey and charcoal gray sweats. "I'm just glad I'm past that part. I'm very humble right now, man, it's a very humbling experience. Just getting a chance to sit back and better myself as a person, as a player, as a teammate. ... I feel like going through that it made me look at the world in a different way, look at my teammates a different way. Hopefully it will work out for the best."

Singletary called it a "fair deal for both parties involved."

The former Texas Tech star was the only draft pick who hadn't signed; even rapper MC Hammer got involved to finally make it happen.