I honestly don't think we've seen the full potential of Chen. This year he hasn't looked too impressive, but he's only pitched in three starts. In 2012, his first year in the majors, he pitched to a 3.64 ERA from April to July before fading down the stretch. He pitched to a 4.75 ERA in August and September. Fatigue should be expected, as Japanese pitchers are used to the six man rotation. I think Buck should have kept giving Chen an extra day off in 2012, when the schedule allowed it. It would have been beneficial to have him pitch on five days rest as much as possible in 2012. He could have built up endurance during his first full off-season with the club the 2012/2013 winter.
I don't believe Chen really got a fair chance at 2013. His oblique injury appeared right in the heart of the season, causing him to miss a little bit of May and July, and all of June. Again, he had an awful August and September in 2013 too. Maybe he was just never the same after the injury. I do believe Chen got a surgery done during the 2013/2014 winter, so maybe he was pitching hurt a little.
It's a possibility Chen has finally built up his endurance in his third year of major league baseball and it appears he is now completely healthy. Look for a season where his ERA is finally under 4.00.