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A player is returned to play in two stages. First, he is cleared by the attending physician, in this case the orthopedic surgeon that both performed the surgery and oversaw the rehab. Second, the player will be cleared by the team, usually involving both the medical and coaching staffs. This clearance means that they both feel it is the proper time for the player to return to play.
The first clearance is the most important, of course. The doctor will not clear a player if there are significant physical deficits. The affected area will be tested. In terms of a knee, there will be strength tests measuring the knee against the unaffected knee and against the healthy condition of the knee prior to surgery, if known. Since Rose (the photo shows him on April 25) has been with the Bulls since entering the NBA, it is likely he was examined closely during the pre- and post-draft process, which would have included testing of his knees and other physical traits.
The team's clearance will be more focused on sport-specific function. Instead of objective measures of physical strength, the team will look at subjective abilities of running, jumping and playing basketball. Since this is a much fuzzier process, much more weight is given the to doctor's clearance.
It should be noted that at most points, the athlete is driving the clearance, asking the team to get out on the floor. Teams often have to "tug the reins" to slow an athlete, hoping to prevent re-injury from the athlete doing too much too soon.