2010 was Jeter's worst season of baseball, and up until he strained his calf in June of 2011, the shortstop was hitting just .260/.324/.324 in 293 plate appearances. During his recovery for that calf, Jeter worked out in Tampa with an old hitting coach, Gary Denbo. That mid-season DL stint was a miracle in disguise, as Jeter's swing improved and he finished the season by hitting .331/.384/.447. That success carried over to 2012, and Jeter's career was given a new breathe of life.
While that DL stint will be remembered for his career rebound, it should also mark the beginning of the end for his health. After this calf strain, Jeter struggled late in the year with his knee, then in 2012 he again had calf tightness. In 2012 he struggled for a good portion of the year with an ankle contusion, which lead to an ankle fracture during the playoffs. In Spring Training of 2013, Jeter re-injured that fracture while struggling with inflammation in his foot. In July, Jeter tried to return but strained his thigh, which was followed by yet another calf strain, and finally an injury to his other ankle. Jeter's legs haven't been healthy since that DL stint in June of 2011, and they may never be healthy again.
Yet the Captain finds himself as the starting shortstop in 2014, perhaps the most demanding position on the field for your lower body. He's had an offseason to recover his legs, but not only have injury questions surrounded him, age questions have continued. In his age 40 season in 2014, Jeter could set a precedent for the amount of innings a shortstop can handle at the position, but he may also prove that 40-year old shortstops shouldn't be counted on. It might be a negative portrait to paint for the great Yankee, but it's also a realistic one.
Regardless of the questions, Jeter found a way to defy old age up until his 2013 season. With Andy Pettitte and Mariano Rivera pitching past age 40, it's possible that advances in medicine, nutrition, and conditioning have extended the longevity of modern baseball players. For the last few years, the Yankees have proved that 40 is the new 30, that older players are still capable of being elite impactors.
Yankee fans and the organization have to hope that this optimistic narrative is true. Jeter has spent an entire career conditioning his body to play through his 40's, and 2014 success could be viewed as a long shot, the Yankees have 12 million reasons to believe that still has some contributions left in him. Wherever you land on Jeter's 2014 projection, as an optimist, realist, or pessimist, projecting him at all will assume that he's at least playing baseball.
As I mentioned above, Jeter's 2011 season was split between two different swings, and his trends are therefore tough to identify. 2012 is probably the most efficient sample size of data to examine how a 40-year old Jeter will perform. Some of these trends include a decrease in strikeouts and walks. According to PITCHf/x, the shortstop is swinging at more pitches than ever, and he's also making more contact. That type of contact includes a steady stream of line drives, as well as an increase in ground balls.
Generally, for any baseball player, an increase in ground balls means an increase in batting average and decrease in power. Unless you're a serious power hitter, ground balls are more likely to become hits than fly balls, thus why we see increases in batting average for players that hit more ground balls. But for Jeter, a player that's had a very poor recent history with his lower body, ground balls could be a problem. If Jeter's speed has been affected by his ankle, calf, or thigh problems, we'll see fewer infield singles, and less of him stretching singles into doubles.
Jeter's style of play relies heavily on his legs, so not only is there the risk of decline in his batting average, but there's also a risk of re-injury as he continues to rely on his speed. Unless we see him take a vastly different approach at the plate, Jeter will see a significant decrease in his batting average from 2012, and I'd peg his final slash around .280/.330/.390. The bigger question will be the number of plate appearances, and unfortunately I don't see him staying on the field for more than 400 of them. Of course, Jeter has the capability of defying age yet again, but I can't gamble on that, not after this lost season.