Although Lester’s ERA is 4.53, other statistics show that a large part of that figure can be attributed to bad luck.
FIP—fielding independent pitching—is a metric that measures a pitcher's success by only what he can control: home runs, walks and strikeouts. It’s calibrated to be on the same scale as ERA.
For his career, Lester has an ERA of 3.63 and a FIP of 3.65—right in line with what they should be. But this season, his FIP is 3.68, suggesting that he’s pitching better than his ERA indicates.
xFIP is a similar statistic that is essentially the same as FIP, expect for the fact that it takes a pitchers HR/FB ratio and puts it to league average, and then calculates a figure in line with ERA. It’s meant to measure if a pitcher has been more susceptible than usual to home runs.
For his career Lester’s xFIP is 3.71, and this year it’s 3.65. With that number one would expect a similarly low ERA, but that hasn’t been the case so far.
As Lester continue the season, look for those numbers to normalize and for him to reel off a stretch of great starts.