Injuries are part of the game. They happen every season and they happen on every team. When the unfortunate occurence takes place, the next person on the depth chart has to step up and accept the challenge of being a suitable replacement.

Mario Manningham knows it for a fact.

The sixth-year wideout is recovering from a late-season knee injury, but is optimistic about the 49ers being able to fill in for the play-making ability of Michael Crabtree.

San Francisco's leading wideout will be out for a bulk of the 2013 season after tearing an Achilles tendon, but Manningham knows the NFL when it comes to injury. When Manningham was injured late in the year, Crabtree took on an even greater play-making role on the perimeter.

"It's sad to see somebody get hurt that's a great value to our team," Manningham recently told SiriusXM radio, "but the next person has to step up. We all know injuries are a part of the game."

Manningham battled a shoulder injury during his first season with the 49ers and was lost for the year in Week 16. The former Super Bowl hero for the New York Giants was inactive for three games during 2013, but caught 42 passes for 449 yards and one touchdown. Manningham missed a week 7 win over Seattle and then played the next five weeks of the regular season before re-injuring the shoulder in a road loss to St. Louis.

Manningham was inactive for two wins over Miami and New England and returned to action in a Week 16 loss to Seattle. In that game, the 49ers wideout took a helmet-to-knee hit that put him on injured reserve a week later. It was unfortunate because of Manningham's rapidly growing rapport with Colin Kaepernick.

Despite the bitter end to his fifth year in the NFL, Manningham has been rehabbing his injury at 49ers headquarters this offseason. The athletic wideout known for his deep sideline receptions has been in good spirits about the setback he suffered in his first year in San Francisco.

Like running back Kendall Hunter and fellow wide receiver Kyle Williams, two key play-makers who were out of action for the 2012 postseason, Manningham's injury recovery has been going just fine.

All three players have been able to steadily progress this offseason and intend on making big plays for the 49ers this coming season.

"I have started and running cutting and doing little things," Manningham said, a key development in his recovery.

The 49ers wideout has been working on strengthening his injured knee all offseason with a cautious approach. Manningham understands that his injury rehabilitation is crucial. The 49ers wideout is taking a calculated approach so he can be most effective for his team.

"When you have knee injuries, you can't really take any time off," the 6-foot, 185-pound wideout said. "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 didn't disclose an exact return date – he's waiting until he gets full clearance to rejoin his 49ers teammates.

"Whenever God wants me to come out and play, then when I'm 100 (percent)," Manningham said. "That's when I'm going to go out there."

Jim Harbaugh is optimistic about Manningham's recovery.

"Mario Manningham (is) probably a little bit later than Kyle," the 49ers coach said, "but he's on track to be healed up and ready to go, as well."