The 27 year old went 5-5 with a 3.13 ERA, 0.957 WHIP, and a 2.71 FIP in 70 games out of the Athletics' bullpen in 2013. He allowed only 2 of his 32 inherited runners (6%) to score. This was in addition to 26 holds and 2 saves in 7 opportunities. He was mostly the setup man for closer Grant Balfour, whom is now with the Tampa Bay Rays.
So far this season, Doolittle has posted a 3.12 ERA, 0.808 WHIP, and a 3.98 FIP in 8 appearances. Over his career, Doolittle has a 3.10 ERA, 0.992 WHIP, and a 2.56 FIP. He has been a constant force out of the 'pen. The metrics agree. Doolittle has a 123 ERA+ and a 2.2 WAR over his career as well.
Jim Johnson has been struggling so far this season, so it will be interesting to see if Doolittle will get the closer's job at some point. He is definitely second in line, but he has not fared well from the position in the past. By signing this contract, both the Athletics and Doolittle will not worry about arbitration and continue playing baseball.