Flexen was also a 25/26 year old pitching in the equivalent of AA or AAA. He should be expected to do well. Put another way, I'm sure if you busted Randy Dobnak down to Wichita and left him there the whole year, he'd probably win that league's equivalent of the Cy Young. That doesn't all of the sudden make him a great MLB option.
Further, if you look at Flexen's numbers game by game, it's not like the one disaster start is his only bad start. By FIP, he had bad starts against Detroit, Oakland, San Diego, Texas, Anaheim, and the Twins (minimum FIP in those 6 is 4.83, with 3 of those starts above 5 FIP). He's given up 4 ER or more in 4 of his 12 starts, and 3 ER in another 2 (and one of those was only a 4 inning start). In reality, Flexen has been feast or famine, with 6 very good starts, one average start, 4 bad starts, and one horrible start. With his low k rate (15.9%, league average is 24%), there is a lot of potential for regression here.