Many reasons for Don Mattingly's big year.

If you are a young Joe DiMaggio or Mickey Mantle with a swing that belongs in the Louvre, somebody might get the idea that you could be a threat to win a batting title, even if it is only your second year with the New York Yankees.

But if you are Don Mattingly and report for spring training with only 163 days of major league service on your record, your first priority (at least in your own mind) is apt to center on just staying with the ball club. The fact that management hasn't decided yet whether you best fit the team as an outfielder or first baseman is strictly its problem.

''There never was any doubt that Mattingly would play regularly for us somewhere,'' explained Manager Yogi Berra about the young man who has been battling teammate Dave Winfield all year for the American League batting title. ''He's one of those kids who came in and adjusted to big league pitching right away. You don't explain it, you're just happy when it happens to somebody on your ball club.

''Now we're starting to get questions about how a kid who hit only four home runs for us last season is gonna hit five or six times that many this season (he already has 22),'' Berra continued. ''Hey, the kid got smarter, grew a little more physically, and then we helped his power by getting him to hit more off his back foot.

''In the spring I planned to keep him in my lineup every day, but swing him between the outfield and first base. Then we have some injuries and I have to leave him at first base and he's doin' the job for me so I don't change him back. It happens all the time in baseball.''

Although Berra may have been the first to suggest some minor mechanical changes in Mattingly's swing, it was Yankee batting coach Lou Piniella who actually did the teaching.

''Basically, Don is what I call a line-drive hitter with power, only last year he wasn't taking advantage of that power,'' Piniella said. ''We weren't after any big makeover when we adjusted his stance. We like the way this kid sprays the ball to all fields and makes things tough on the pitcher. But over the season a hitter is going to get a certain number of balls that he can drive for home runs, and we wanted him ready to take advantage of that situation.

''The best part of Mattingly is all the little things he does instinctively as a hitter, things that can be talked about but not really taught,'' Lou added. ''I like him because he's the kind of hitter who can go 0-for-3 against a good pitcher and then beat that same pitcher with an extra-base hit his next time up.''

Part of that is talent, of course, but another part can be attributed to Don's serious approach to his craft and his willingness to work hard at it. When he was in the minors and got into a batting slump, for instance, he would try to catch a California Angels game on TV so he could study the smooth swing and bat rhythm of Rod Carew.

Mattingly is one of those players who doesn't hit the ball so much as he stings it. Like Carew, he'll lay the ball just inside either foul line for extra bases, and so many of his hits skip past diving infielders that you're convinced the ball had eyes.

And with his average now hovering around the .350 mark, those big league scouts who shied away from him because of what they considered below-average foot speed for an outfielder are undoubtedly wishing now that they had paid more attention to his anticipation and quick bat.

Off the field, the 23-year-old Mattingly is so soft spoken, polite, and down-to-earth that the aggressiveness he brings to the plate doesn't surface at all in interviews.

Questioned about his experiences in Puerto Rico, where he played winter ball and won the batting title with a .368 average, Don said the thing that actually pleased him more was the unexpected extra practice he got hitting against left-handers.

''Last winter, for some reason, there was an unusually large number of southpaws pitching winter baseball, and having the ball thrown at me from that angle was something I needed,'' Mattingly (a left-handed batter) told me. ''Because of what I was able to work on during the winter, American League southpaws haven't seemed nearly as tough to me this season.

''As for my increase in power, Piniella got me doing two things differently in spring training that really helped me. First he got me hitting more off my back foot; then he taught me a better way to swing through the ball so that I wasn't pulling up short and losing some of my power. But while I enjoy hitting home runs, I think most of the time I'm better off using the entire field in which to spray the ball.''

Asked if first base now belonged exclusively to Mattingly, Berra replied: ''It don't matter where he plays. He can hit, can't he?''