Lance Lynn is off to an incredibly hot start. He has lessened the impact of losing Chris Carpenter until June (at least), as is one half of the reason the Cardinals are 7-0 in starts by him or Kyle Lohse, and only 4-5 in starts by Adam Wainwright, Jaime Garcia, and Jake Westbrook. But it's not going to last.
First, let's start with why Lynn has been successful thus far. The bottom line is he transformed from thrower to pitcher in the offseason and spring training. He added a fourth pitch (his slider) and is throwing his changeup more. Adding that fourth pitch was crucial to him being a successful starting pitcher, because he's not just seeing hitters once anymore. He needs to use his secondary pitches to keep hitters guessing the second and third times through the lineup. He has successfully integrated his secondary pitches into his arsenal, only throwing his fastball 63% of the time this year as opposed to nearly 75% of the time last season. Now, I have no hard evidence to prove this correlation, but adding a fourth pitch is helping him miss bats as well. He is striking out 8.05 batters per 9 innings, and is sporting a 25.4 K%, 10th best in the National League.
However, the other reason he has been so successful is he has been incredibly lucky. He's currently sporting a .182 BABIP (batting average on balls in play). BABIP tries quantify three main things: the quality of the defense behind the pitcher, luck, and overall talent level (if a pitcher is tipping his pitches or his mechanics are off, etc.) Pitchers with higher strikeout rates tend to generate weaker contact on their pitches, so if Lynn can keep up his high strikeout rate, he should be able to keep his BABIP under the league average. To give you some context, league average is generally around .290-.300. Justin Verlander has a career BABIP of .284, Adam Wainwright a .288. He also has an astronomical 98.2% strand rate (or, Left On Base %), second best in the majors. He should start to experience a regression to the mean sooner rather than later, so look for Lynn to start allowing more base runners, and for those runners to score once they reach base.
The metrics bear it out, too. His FIP (fielding independent pitching, or, what his ERA is independent of defense and other things out of his control) is 3.15 in contrast to his 1.42 ERA. Now, this is not to say that Lynn will suddenly come apart, actually quite the opposite. His FIP is still well above league average and, if you noticed, his xFIP (expected fielding independent pitching, or, what his ERA should be independent of defense and other things out of his contorl) is an even better 3.11. When Lynn starts to regress, he shouldn't regress back to a bullpen arm, but rather around the level of a quality third starter.
What's most interesting about Lynn is that he doesn't really know what kind of pitcher he is yet. He's definitely a fly ball pitcher (41.3 FB%), but he seems to want to be a ground ball pitcher as well (45.7 GB%). But he's also a strikeout pitcher (25.4 K%). He should probably concentrate on keeping the ball lower in the strike zone and inducing more ground balls, because his HR/FB percent has always been poor (11.5% last season, 10.5% this season). If he's going to continue to be unable to keep fly balls in the park, it's probably in his best interest to keep the ball on the ground (a true ground ball pitcher usually has a GB% upwards of 50%) and continue to strike batters out at a good rate. By reducing the number of fly balls batters hit off of him, Lynn is going to keep his HR totals down, which will help him as he regresses back to a more normal rate with some of his "luck" numbers (BABIP, LOB%, etc.)
Bottom line on Lynn is that he grew into a pitcher this last offseason. His added fourth pitch will help him continue to strike out batters, but he has been extremely lucky. If he can reduce his FB% and increase his GB%, he could end up being a quality number 3 starter for the Cardinals by the end of the year. Lynn's emergence has given the Cardinals valuable time to allow Chis Carpenter time to properly rehab his shoulder, instead of trying to rush him back (the man's 36 years old, after all). By the time he does come back, the Cardinals may have to look for a different candidate to move out of the rotation. Also, depending on how far along Shelby Miller is in his development, the Cardinals could look to trade off some excess pitching or hitting (they have a ton of bats when everyone is healthy, but that's a different post for a different day) to pick up an impact bat or shore up the bullpen. But at only 23, I don't think the kid is going anywhere for a while.