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A utility infielder (UI) is a baseball player, usually one who does not have a regular starting role on the team and who is capable of playing more than one of the four defensive infield positions: second base, third base, shortstop, and less typically first base. Utility infielders are generally considered excellent defensive players who do not hit well enough to remain in the starting lineup, but can fill in at multiple defensive positions to give the various starters a rest, or replace a starter late in a game to provide improved defense when the team is winning.
Examples of current utility infielders include Geoff Blum, John McDonald, and Jayson Nix.