If it's clear that Lee is the better defensive SS and Castro's offense wouldn't be adversely affected by a move to 2B, then having Lee play SS would clearly be the best move for the team. If Lee struggles offensively after being called up, there's no reason why they couldn't just move Castro back to SS and send Lee to AAA. Changing positions does carry with it a heightened injury risk. So if you're gonna make a move, you only want to make it once. There is no reason to move castro off short, move him back when Lee struggles, move him off again when Lee gets another shot, and so on and so forth until we know whether Lee will be a ML caliber hitter. I guess I can buy some of what you are saying, but there can't be that much injury risk of having Castro go from short to second and back to short. Think about turning the DP in a hurry. Momentum is taking each player in different ways, requiring not just the use of different major muscle groups, but different stabilizing muscles, tendons, ligaments, etc... Think about it this way. If MLB came out tomorrow and said that the order of all the bases had been reversed, that everybody now goes from home to 3B to 2B to 1B and then back around to home... how many people do you think would blow out their knees and ankles in the first couple weeks? Every big fat right handed batter who has to run backward would. 99% of all such injuries would be as a result of baserunning in the opposite direction that you have done all your life. I think you are greatly dramatizing the danger. It's different, sure, but it's not asking a LF to play catcher or a 3B to play CF. Middle infielders routinely play one or the other position. I don't want him switching midseason if possible. But if it comes down to it and he starts spring training at 2B, and then the next season has to move back to short, I think the risks are extremely minimal.