Not a good sign. I don't think the boss is sincere enough. He wants the lab manager to pre- select for him. If you join this lab, you not only need to satisfy him, but also his lab manager. I think mainly the lab manager would ask: why you want to apply for this lab? What qualification do you have to contribute to the lab? What project/direction are you interested in exploring? Good luck. In the mean time, try to find other labs.