Joe Girardi didn’t want to mess with Kevin Youkilis’ back after watching him slide into first base to record an out Saturday. So the manager started Jayson Nix at third and returned Mark Teixeira to first yesterday for the Yankees’ 2-1 win over the Mariners after he was the designated hitter in Saturday’s 3-1 win. “He was a little stiff. I was probably going to give him a day off [yesterday] anyway. It was going to be [Saturday or yesterday],’’ Girardi said. “This way he gets back-to-back days. His stiffness probably has something to do with him playing five days in a row.’’ The Yankees are off today and open a three-game series in Oakland tomorrow night. There were several late-game pinch-hitting spots in which Girardi could have used Youkilis yesterday, but he didn’t. “I was trying to stay away from him,’’Girardi said. Girardi has a reason to be cautious with Youkilis. On April 20, Youkilis came out of a game in Toronto with what he said was nothing more than tightness in his lower back. Seven days later he played again and then went on the disabled list from April 30 to May 31 with a strained lumbar. In the second inning of Saturday’s game, Youkilis fielded a Raul Ibanez grounder and decided the only way he was going to get an out was to slide feet-first into the base. Youkilis believed that was the best way because he didn’t know if Andy Pettitte was going to make it ahead of the heavy-legged Ibanez. “Half the problem is I go as hard as I can,’’ Youkilis said. “Maybe I’ll dive head first. [Saturday] it was stiff but I will be stiff the rest of my life.’’