Now that Michael Crabtree is joining the Raiders, what does it mean for the team?
He is an upgrade at wide receiver
Crabtree instantly becomes a starting caliber receiver in this offense. He would likely line up opposite Rod Streater with James Jones working in the slot. Then as for the current roster, Andre Holmes, Brice Butler, and Kenbrell Thompkins would battle for the remaining spot(s).
He's not a surefire number one receiver
He has the potential to be. He always has had that potential but has only once come close to putting it together. He has one season over 1000 yards. That was 2 years ago when he caught 85 passes for 1105 yards and 9 touchdowns. The Raiders struck out trying to fill the number one receiver role when Randall Cobb opted to return to Green Bay.
They will still look to draft receiver
Adding Crabtree is not the answer. He doesn't preclude the team from drafting a wide receiver in the top two rounds any more than the Raiders trading for Matt Schaub kept them from drafting a quarterback in 2014. It simply relieves them from reaching out of desperation with the fourth overall pick. I still expect wide receiver to be taken in the first two rounds, perhaps even at fourth overall.
He's not a long term solution
The details of his contract have not been released as of yet, though it is rumored to be a one-year deal. He wore out his welcome in San Francisco and the Raiders wouldn't saddle themselves with a risky player long term. There was a reason he was a free agent for over a month and the best reported offer he received was for $3 million per season.