Nathan says he felt fine while he was working out before spring training games. But there was some soreness when he threw 20 pitches against the Boston Red Sox on Saturday.
Nathan was expected to fly back to Minnesota on Sunday for tests. The 35-year-old right-hander had surgery Oct. 20 to remove bone spurs from the elbow.
Nathan says he isn't too worried, and will know more after doctors compare tests results with those taken last year.
Nathan was 2-2 with a 2.10 ERA with a career-high 47 saves last year.