The Angels are in the middle of another under whelming season and thus far, Frieri's early struggles have contributed to this. It was just a few weeks ago that those same struggles cost him his closing job (which he is getting back just now). The question to ask yourself is whether we'll see the Frieri going forward that pitched very strongly in his first five seasons with the Padres & Angels that still has a 128 Career ERA+ even counting his early struggles or is his 6.08 ERA/6.49 FIP more indicative of his career starting to tumble towards its end even at the age of 28.
Due to his FIP indicating that his scary numbers are about where they deserve to be so far and an opposing BABIP of .290 indicating that his struggles aren't the result of bad luck, I would shy away from Frieri unless you are absolutely desperate for saves.
Ironically enough, his most similar player through age 27 Bryan Harvey was also a closer for the Angels. While he had a couple monster seasons after turning 27 (1991 & 1993) his career was over in his early 30s (though in his case it was due to injuries and not an ever expanding FIP.
Whenever we have ourselves a little heralded starting pitcher with a 2.41 ERA/155 ERA+ in early-May, naturally I am a bit skeptical. In his first extended look last year, he had his struggles (88 ERA+/1.357 WHIP) but since this was done with the Marlins, it was barely noticed.
This appears to be a case where you should let someone else buy high on Koehler. His FIP is at 4.40 which is actually a little bit worse than it was last year and an opposing BABIP of .217 indicates that he's more the beneficiary of good luck versus being a breakout fantasy candidate.
Well, I guess somebody has to win this battle. Honestly, i'm not even close to being sold on either of the above options. While Koehler has had a nice stretch to start the season, it's only nice from a distance as a lot of his shiny numbers are the result of smoke and mirrors. While Frieri has had some success in past seasons, it appears that his struggles are a sign of trouble ahead. Since Frieri has a stronger history (even if said past success has come and gone) combined with relative youth (28), he wins this battle because somebody has to.
Winner: Ernesto Frieri
All numbers, comparisons, etc. found on baseball-reference.com