Win, I don't see an issue with Manningham and Boldin being a trend. Manningham was doing very well for us until he got hurt. He wasn't a bust by any means. I'm hoping he can come back, but we can't count on it. We now have Boldin, we should draft a couple receivers, and then we hope that Jenkins develops, and Williams and Manningham come back. Out of all those guys we ought to be able to develop a receiving corps that is formidable.

replacing Walker will be a tougher thing to do because of his ability to block. My idea is you look to the same pool we got him from: "non-burner" wide receivers who are tough. Walker is 6'1 1/4", 242 (230 when he came out), ran a 4.5. Here is what Scout.com said about him when he was picked:
Despite his imposing physical tools, Walker is a raw prospect who needs work on his hands and route-running skills. Walker clearly a good athlete and has significant upside to become a contributing receiver, and perhaps even a possession-type starter. He must improve his hands and pass-catching skills to succeed at the NFL level, but he can contribute immediately on special teams while he develops.
So who are the guys who fit this mold? There are bound to be some people out there.