Tom Tango and Marcel have a simple answer. They would pitch 25 innings with an ERA of 4.32, which is the default forecast for a reliever with no major league experience. Were Major League Equivalents still available, we might get a different answer. Looking just at Watson, he had a terrific 2010, capping it with ten strikeouts over six innings in the final game of the EL playoffs. (Moskos saved the game with a two strikeout ninth.) But most of the time he was a reliever, chalking up an ERA of 1.84 with only 39 hits, 12 walks, and 66 strikeouts in just under 64 innings. His ERA against lefties was 1.38. This is not entirely chopped liver.
At Altoona Moskos was also lights out. He had an ERA of 1.52 with only 26 hits, 16 walks, and 43 strikeouts in just over 41 innings. Against lefties his ERA was 1.20. Again, one could do worse. Of course the experience in AAA was less than thrilling.
So what are the chances that a healthy Beimel would rack up an ERA of under 4.32? His Marcel ERA is 3.91. I'd pick Watson if I were picking. What about you?