James McDonald, formerly considered a reliever caliber pitcher, got off to a promising beginning as a starter in 2010 with a six inning shut out against the—then—middling Colorado Rockies. He didn't do so well for the rest of August, but pitched to a fine 2.31 ERA in September (courtesy of ESPN).
Given this backdrop, McDonald got off to a disappointing start in 2011, his sophomore year, with an April ERA of 7.66. But he was actually very good for the next four months of last year. The ERAs for the middle four months were 2.86, 3.76, 3.14 and 3.93. His cumulative ERA had dropped into the 4's until a weakish September ERA of 4.93 pulled it back up again.
McDonald pitched like an ace in 2012, until the All Star game that is, with a front of rotation ERA of 2.37. Then he went on a deep slump for six games in a little over a month, giving up 30 runs in 31 innings for an ERA of 8,71 over that stretch.
He seems to have righted himself in the last two games against St. Louis, giving up no runs in 13 innings of work, although this was interspersed by a four-run five-inning losing effort against the San Diego Padres.
In this regard, McDonald seems like a more stable version of a former Pirate "ace" named Paul Maholm. Over the course of a whole season, Maholm's ERA would typically put him in the lower reaches of second starter, or maybe the top level of third starter. But this would be due to one to two bad months (out of six), which worsened his average. Over the remaining 4-5 months, he actually put up ace-like numbers. So the main thing that distinguished him on the downside from a true ace was not ability, but consistency.
The 27-year-old McDonald is still young, and his ability to start, rather than relieve, came as something of a surprise. This was certainly true of the Los Angeles Dodgers, who traded him (and outfield prospect Andrew Lambo) for reliever Octavio Dotel.
If he maintains is current level, pitching well for most but not all of a season, he will be more of a Maholm-type second starter, with ace-like ability most of the time, but not a true ace.
However, if he can localize his bad stretches to individual games, he will become the ace that the Pirates have been seeking for a long time.