Mario Manningham paused from his own injury rehabilitation Thursday to rally 49ers teammates in the wake of Michael Crabtree's Achilles tear."It's sad to see somebody get hurt that's a great value to our team," Manningham told SiriusXM NFL Radio, "but the next person has to step up, man. We all know injuries are a part of the game."Manningham sustained a season-ending left knee injury Dec. 23 at Seattle, and he's been constantly rehabilitating from that anterior cruciate ligament tear."I have started and running cutting and doing little things," Manningham said. "When you have knee injuries, you can't really take any time off. Every time I think about it, I'm trying to do something with my knee. I'm not rushing it but I am going hard on my knee."Manningham, the 49ers' second-leading receiver last year, is not expected to be ready for the start of training camp in two months. The 49ers open defense of their NFC title Sept. 8 against the Green Bay Packers at Candlestick Park.Rather than announce a timetable for his return, Manningham said: "Whenever God wants me to come out and play, then when I'm 100 (percent), that's when I'm going to go out there."Crabtree is likely out for at least the next five months. The 49ers are currently without three of their top four receivers, with only Anquan Boldin fully healthy while Crabtree, Manningham and Kyle Williams (knee) rehabilitate their injuries.