Alden Gonzalez' article on Berkman's rehab explains that Berkman, while he can "barely walk", still thinks it's within the realm of possibility that he could be ready for Opening Day:
The 34-year-old first baseman believes it's "still within the realm of possibility" that he can be ready by Opening Day, which for the Astros is April 5 against the Giants. But before he can progress from simple leg raises and quad-tightening exercises, Berkman is waiting for the swelling in his knee to go down, and he estimated it's about 10 days post-surgery before that happens.
Okay. So...ten days post-surgery means that it'll be next Wednesday before he can do anything involving his lower body. Next Wednesday is -12 days to Opening Day. Can Berkman do it? I'm sure he'll try. Is it realistic? I don't know. I've never had knee surgery. But I do think it's unlikely that the Astros will push him to be ready for the Giants.
We're going to have to see. I can't sit here and say he's going to need three games, four games, a week. You can't say that, because you don't know exactly how he's going to feel, how he's going to heal and how the knee is going to be going. So we're going to just have to wait and see with how it goes and make sure he's 100 percent, both physically and baseball-wise, if he's ready to play."