For whatever reason, sometimes I am only able to find one card of certain players. To be considered a One-Card Wonder, a player must have been active since 1991 (when I started paying attention).
Well, I cannot include a scan of this, because I can not find one, but it's a 1996 Donruss card. I think McGee may have some parallel versions of this one and possibly one or two other cards out there of him, but I have no idea why there are so few Willie McGee Red Sox cards.
Willie McGee was a Red Sox only for the 1995 season, playing 67 games as a backup outfielder. He batted .285/.311/.400/.711 with two home runs and five stolen bases. The longtime Cardinals great was well past his prime when he appeared for Boston, but he played reasonably well still in 95. This was McGee's only full season in the American League though and he returned to St. Louis the next year. I am still working on finding more cards of the former great.
EDIT: I recently acquired a 1995 Upper Deck extended card of McGee. He no longer qualifies for a One-Card Wonder.