Complex sentence: He had originally signed to play collegiately for Tony Gwynn at San Diego State, but signed with the White Sox after they chose him in the 2nd round of the 2003 MLB draft.
Split into simpler sentences: He had originally signed to play collegiately for Tony Gwynn at San Diego State. However, he signed with the White Sox as he was taken in the 2nd round of the 2003 MLB Draft.