How will Mike Moustakas bounce back from his knee injury?

Last May, the Royals' chances of competing took a major blow when Mike Moustakas collided with Alex Gordon while pursuing a foul ball in Chicago. The collision would keep Alex Gordon out of action for nearly a month, and would put Moustakas on the shelf for the rest of the season with a torn anterior cruciate ligament (ACL) in his right knee. Moustakas underwent knee surgery in June with the expectation he would return ready for the 2017 season.

But how will his knee respond?

Mike Moustakas said his knee is "almost 100 percent." Shouldn't have limitations in spring. But'll likely have slower schedule than usual.

— Rustin Dodd (@rustindodd) January 27, 2017

The ACL is the ligament that stabilizes the knee joint. You can see how it fits into the knee here. ACL injuries are most commonly associated with contact sports such as football or basketball, but they have happened with some frequency in baseball as well. The injury most commonly occurs when the knee overextends and twists, causing a tear in the ligament. Surgery typically requires a graft or cadaver ligament to replace the torn ACL. Months of rehabilitation and therapy are required to get the knee back in game condition.

Here is the brace Moustakas is wearing in spring training.

A limited sample study in the medical journal Arthroscopy concluded that Major League players play, on average 20% fewer games after their ACL injury. The injury may affect performance with a 12.3% decline in batting average for players that suffered an injury to their rear leg. Those with injuries to their front leg actually had a 6% increase in batting average. However with a sample size of just 26 players, and with the use of batting average as a metric of performance, it is not entirely clear this study will reveal much about the impact the injury will have on Moustakas.

Moustakas injured his right knee, on his front, or "plant" leg when he is hitting.

Cubs left-handed hitter Kyle Schwarber injured the same knee the first week of the season, and was able to return for the post-season for a few at-bats, although it still remains to be seen how his knee will react in the field. Nationals catcher Wilson Ramos and Rangers outfielder Josh Hamilton also suffered ACL injuries last season, and Hamilton has already been sent to the doctor to have his knee examined.

Aside from Schwarber, perhaps the most high profile ACL injury from a hitter recently was when Tigers designated hitter Victor Martinez injured his knee before the 2012 season. Martinez was coming off a terrific run the previous few seasons, and had hit .330/.380/.470 with 12 home runs the previous season with Detroit. Martinez, a switch-hitter, injured his left knee.

However, Victor Martinez is a switch-hitter, who injured his left knee, which would be his front, or "plant" knee when batting as a right-handed hitter against left-handed pitchers. We can note a significant decline in his power numbers as a right-handed hitter following surgery, although curiously, his hard-hit rate increases sharply.

Martinez actually had his highest amount of soft contact and lowest amount of hard contact in 2011, the season before his injury. After the 2013 season, his power seemed to increase, going from 12 home runs in 2011 to 32 in 2014, with higher rates of hard contact. Long-term, he appears to have bounced back, although he did have a setback in 2014 when he tore his meniscus in the same knee.

Other players have not been so lucky. An ACL injury pretty much ended the career of infielder Scott Sizmore, just as he was on the brink of landing a starting job. Mat Gamel was once a promising Brewers prospect, but after inconsistent performance and an ACL injury in 2012, his days in affiliated ball were over at age 26. Knee injuries to Rays prospects Tim Beckham and Hak-Ju Lee may have hurt their development as Top 100 prospects. Post-season hero Aaron Boone was released by the Yankees after he had an ACL injury sustained in a pickup basketball game in 2004 and was never quite the same after that.

Mike Moustakas seems to be playing on a higher level of performance since changing his hitting approach before the 2015 season. If you pro-rate his numbers over the last two seasons to 162 games, he is hitting .277/.340/.475 with a 117 OPS+ and 27 home runs. If you pair that with his fantastic defense, you get quite possibly the most valuable player in the Royals lineup. If the Royals hope to contend in 2017, they will need a healthy Mike Moustakas. We will all be waiting with bated breath to see how that knee responds the first time he plants that right leg to make a throw across the diamond.