I may have a solution. Make a Two-Way Player and load him into the RTTS save that's giving you the error message. Then simulate until you're called into the Manager's office for poor batting performance. When this happens you are given the choice to be sent down to AAA or to only be a pitcher. When you choose to only be a pitcher you then manually save and exit to the main menu where you can change back to your pitching player. Finally, load back into your RTTS and the error message should be gone.
P.S: You may have to sim multiple times because if your pitching is poor while simulating they'll send you down to AAA. Also, I have only pitched one game so the error may just come up again. Hope this helps though 