Gary Sheffield worked out on Monday with a host of Yanks coaches as he goes through a crash course on how to handle first base. Hitting coach Don Mattingly, who won nine Gold Gloves at first base, was the primary teacher and said afterward that he was impressed with Sheffield's prowess. "It looked easy for him," Mattingly said. "He can do it. There's no question he can do it." Sheffield believes the move will "enhance his value" as a player - something which could prove important since he may be shopping for a new team next year if the Yanks don't exercise his $13 million option.