The five-year extension the Cardinals gave second baseman Kolten Wong last Wednesday sets up the 25-year-old financially and provides St. Louis with some cost certainty, plus the chance to lock up two of his free-agent years, including a club option. It's the type of deal that makes sense for both sides.
Over the past two seasons, Wong has produced a solid 4.3 wins above replacement, according to baseball-reference.com, ranking 14th among MLB second basemen. While he doesn't necessarily have to improve on that performance in order to justify the contract, there is reason to think he could do so, with Cardinals manager Mike Matheny saying he doesn't believe Wong has "begun to tap into what kind of player he can be." Now, with shortstop Jhonny Peralta in line to miss significant time with a thumb injury, there could be more pressure on Wong to do just that.
• Wong agrees to five-year extension
There are multiple ways Wong could raise his game, but one would be for the left-handed batter to become more proficient against left-handed pitching. As both traditional numbers and Statcast™ data suggest, Wong has a ways to go in that area.
Career vs. RHP: 847 plate appearances, .251/.309/.390 (.699 OPS)
Career vs. LHP: 261 plate appearances, .248/.282/.325 (.607 OPS)
Wong actually performed better against southpaws in 2014, though that was in only 76 plate appearances. He got more than 100 additional chances last year -- still a small sample size -- and posted a .552 OPS, fifth-lowest for all Major League hitters with at least 150 plate appearances in those situations. It also was more than 200 points below his mark against righties (.772). However, Wong told MLB.com that he isn't particularly concerned with his splits.
"I feel like I do pretty well against left-handed pitchers," said Wong, who handled them well in the Minors (.753 OPS). "I'm going to continue to do what I do against lefties. I don't really pay attention to that too much. I feel like I have a pretty good idea of what I want to do against lefties. Maybe [my OPS] was down last year, but most of the time when I faced lefties I did what I needed to do."

However, Statcast™ further illustrates the issues Wong faced in 2015. Against righties, his average exit velocity was 89.5 mph on 295 tracked balls in play; against lefties it was 84.1 mph on 105 balls in play, 14th-lowest among lefty batters (minimum 50 balls in play). 
The spray chart below shows Wong's 2015 output in same-side matchups, broken down by exit velocity, with all of the blue dots in the infield representing weak ground balls. That jibes with the fact that Wong's ground-ball rate was much higher against lefties (53.5 percent) than righties (41.3).

Small-sample caveats apply, and exit velocity alone doesn't tell the full story -- launch angle matters, too -- but the number suggests that Wong's platoon splits weren't simply unlucky. 
In fact, the difference in Wong's average exit velocity was fifth-largest among left-handed batters who had at least 50 balls in play tracked against lefties and 100 against righties. Here is a look at the top 10 on that list (actually 11 due to a tie).

Each of those 11 players also had a sizable split in OPS, with eight producing a gap of more than 150 points (Pablo Sandoval gave up switch-hitting last May to bat solely from the left side but has said he will return to it in 2016).
As for Wong, he walked about half as often against southpaws and struck out significantly more, in addition to hitting ground balls more frequently. His isolated power was more than 100 points lower, thanks to a total of six extra-base hits, including one homer. 
The Cardinals acquired infielder Jedd Gyorko this offseason, giving them a right-handed option at second, as well as a potential backup for Peralta. But the extension shows St. Louis' commitment to Wong, and if he demonstrates an improved ability to handle lefties in his age-25 season, that should further establish him as an everyday player in the long term.