When Washington Nationals shortstop Cristian Guzman smacked a tag on the helmet of Cleveland's Josh Barfield in the fifth inning of a game Sunday, none of the 26,413 at RFK Stadium could have known that it was among the last plays Guzman would make this season. Guzman returned to the field, had two more at-bats, and even dressed for the team's ensuing trip in a suit and tie. But before Guzman could leave the park, he was placed on the disabled list. Monday afternoon, he had surgery to repair ligaments in his left thumb. Thus came a devastating end to a season that had been filled with promise for a player who, two years ago, was booed lustily in his home park.