We live in an age of Sabermetrics. Advanced statistics are the way of the future in baseball. There is no doubt about it. However, I believe there is one statistic that is more important than any new Sabermetric statistic.
Earned Run Average.
Call me old fashioned, but I think ERA is the most important. It gives one insight about how a pitcher battles, and ultimately what kind of a chance his team has to win a game when he's on the mound.
Let's look at some simple mathematics.
The average American League team scores 4.29 runs per game. King Felix has an ERA of 3.26 and averages approximately seven innings per start. Therefore, Hernandez allows approximately 2.54 runs every start. Over those same seven innings, an average American League team will score 3.34 runs. Thus, on average, Hernandez is providing his team with 0.8 runs per start.
Mathematically, Hernandez should win every start. Obviously, this isn't true, but it shows that Hernandez's 8-8 win-loss record is not indicative of how well he is pitching. Felix Hernandez's ERA is proof that, on average, he is giving his team a very good chance of winning the game.
Now let's examine ERA by comparing WHIP.
In 2009, Jair Jurrjens posted an ERA of 2.60 and had a WHIP of 1.21. In that same year, Javier Vazquez had an ERA of 2.87, yet had a WHIP of 1.03. Jurrjens was allowing 0.2 more base runners per inning, yet had an ERA of nearly 0.3 lower than Vazquez.
What does this tell us?
Many baseball fans these days would look at Vazquez's WHIP and say he was the better pitcher in 2009. However, I would say Jurrjens was a better battler than Vazquez in 2009 and therefore, the better pitcher. Jurrjens could pitch better under pressure. Ultimately, Jurrjens' lower ERA gives his team a better chance to win, on average.
The Jurrjens-Vazquez example illustrates that ERA contains information on how a pitcher battles. One could say, he knows how to get a double play when he needs it, or can pick up a clutch strikeout.
However you want to put it, ERA gives one insight about a team's chance of winning when a certain pitcher is on the mound.
Special thanks to baseball-reference.com for providing the statistics.