I don't think MacDougal really "deserves" the job at this point, with how poorly he has pitched this season. His 4.24 ERA is acceptable for a rookie pitcher, but if you look deeper at his numbers, the 1.54 WHIP is just inexcusable, and it's likely that he's been lucky to sucessfully convert as many saves as he has, allowing that many baserunners. For as hard as he throws, his strikeout rate is also pretty poor, less than a K/inning is walking 5 per 9 innings, which is just dreadful for a closer. His control is so off that he also seems to bean at least one batter every inning and throw a wild pitch or two every appearance. (for the year, 6 HBP, and 7 WP, which is also not what you want from a closer). I think it's highly likely for his ERA to baloon into the 5.00 plus range before the end of the year.
Yes he has potential and could be a lights out closer, but only when he learns some control. Right now using him as a closer is like sending out a timebomb every close game. I would not be surprised if Leskanic takes the job over before too long.