In 2012 Zach McAllister had solid strikeout and walk rates, but given up too many home runs. His ERA and FIP were an identical 4.24, not impressive looking, but actually a bit better than the averages for all AL starters, which were 4.37 (ERA) and 4.31 (FIP). In 2013 his strikeouts fell and his walks rose, but he cut his home run rate substantially. His ERA was 3.75 and his FIP was 4.03, both better than average.
McAllister turned 26 during the offseason, and seemed well positioned to continue being a touch better than league average for another few seasons, making him a solid #4 starter.
McAllister did very well at the beginning of this season. Through his first five starts, with a 3.14 ERA and a 2.36 FIP. His strikeouts had rebounded a bit, his walks had dropped back off a bit, and he hadn't allowed any home runs in 28.2 innings. Now, obviously that was a very small sample, but still, it was promising. I wasn't suddenly viewing him as a #2 starter or anything, but I definitely felt even better about him as a #4.
On April 30 McAllister was used on only three days' rest, and he had his worst start of the season to that point, walking 4 and allowing his first home run of 2014. He soon had three bad games in a row, including back-to-back starts in which he gave up multiple home runs and didn't record an out after the 2nd inning. McAllister was then placed on the 15-day DL with a sore back. It was six weeks before he pitched for the Indians again.
McAllister made a couple pretty good starts after he returned, then a couple pretty rough ones. He was sent back to Triple-A on August 1st.
Looking through McAllister's page at Brooks Baseball, I don't see anything that really jumps out as a difference between before the short-rest start and after it, or before the DL stint and after it. His velocity did dip a little in that short-rest start, and a little more in the next couple games, but not enough that I think it means anything, especially because it had fully rebounded by his last couple starts before being demoted, which did not go well.
Was the start of this season just a small-sample mirage? To some extent, it probably was, but that doesn't explain why, after more than two years as an average AL starter, McAllister has struggled so much. It's certainly possible that starting on short rest through him off. The results have certainly been different since then. It's also possible that he's been unlucky. his .322 BABIP is a solid jump above the .299 he gave up over the previous two seasons, and his LOB% (or strand rate) this year is only 58.6%, which is the lowest in baseball among the 161 pitchers with 65+ innings so far this season.
LOB% for MLB pitchers as a whole this year is 73.0%, and 85% of the the 161 pitchers mentioned above are within 7% of that. The farthest outlier above is 12.2% above. The farthest outlier below, other than McAllister, is 11.6% below. McAllister is 14.6% below, far farther from average than any other pitcher with as many innings.
LOB% is not entirely luck driven, but luck plays a factor, and as the most extreme outlier on the scale, I think it's reasonable to assume there's been some bad luck for McAllister. That goes a long way towards explaining why, while his ERA is an ugly 5.91, his FIP this season is 4.04, almost identical to what it was a year ago, when his ERA was more than two runs lower.
McAllister is almost certainly never going to be a really good starter, but having looked at his numbers, I think he's better than he's looked during the last two and a half months, and I still think he can be a solid back-of-the-rotation starter.