Jason Giambi third base?

I need to complete my Giambi missions, but he is only eligible at third base, and my third baseman is the best player on my team. I don't want to sub him out for Giambi. Why can't I play Giambi at first base which was his actual position? Am I missing something?

He was 3B as a rookie. It's a rookie card.. hence 3B

Presumably because he didn't become a first basement until ~97 and the bronze is his rookie card, where he'd have been more a 3rd baseman and occasionally outfielder.

Your current 3rd baseman doesn't have 1b as a secondary?