well, I would not say it was a huge problem before. They are second in MLB in pitching WAR, behind the Nats, so they must be doing something right. Their bullpen is incredible right now and can only get stronger when Feliz moves out there. Their ERA is best in the AL, and their FIP is second in the AL. They are not giving up homers despite playing in a homer prone park (OK, any ballpark Hamilton, Beltre, Kinsler, Cruz, Murphy, and Moreland play in will be homer prone).
Lewis, Darvish, Harrison, and Holland all have xFIPs between 3.79 and 3.82. ERAs vary more, and Lewis is a bit homer prone (park I think was set up for lefty power). I like Darvish, Lewis, and Holland a bit more because of the Ks and the K/BB. One of the reasons I don't think Oswalt is a big help for them in the post season is I think those 3 are already very good.