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. I'm not trying to dramatize the danger, I was just using an analogy to make sure the point got across. Something so functionally similar does not mean in any way that your body can handle the different strains placed upon it. So there is certainly at least some degree of danger. Hell, if it played out like you just described, with changes being made in the offseason (and a workout program that can be designed to help with that), I'd have no problem at all. I just don't want the nightmare scenario popping up where Castro is moving between positions every 3 weeks while they decide to give Lee another shot, then send him down, then bring him back up. We need to be pretty damn confident Lee is going to be a ML caliber hitter to justify moving Castro... and that's without even bringing up potential trade value changes and whatnot.