One of the reasons that Brock Lesnar is being used so soon after WrestleMania and that the Undertaker is working a match on RAW next week is because CM Punk won't be returning anytime soon and WWE feels that they need star power on TV.
The original plan for CM Punk was for him to address the crowd on the RAW after Wrestlemania 29. He would whine about his loss and then "walk out" on the company. WWE held Punk off of that show because officials felt that Punk would get a big reaction from the post-WrestleMania RAW crowd.
But there is more to the story. Punk was not happy with the verbiage that WWE wanted him to use, and didn't want to come off as a whiner. A few weeks ago, Punk had a big blow up with the WWE creative team, and it got to the point that he wanted to give his notice to the company. Punk eventually worked out his issues with WWE management, but it was decided then that he wouldn't work live events, instead just TV and WrestleMania 29.
With the knee injury and issues with creative, the decision was made to write Punk off of TV. While there is no set plan for Punk's return, word is that he won't be back until at least SummerSlam time.