I think Castillo would pitch 180 to 200 or so innings that would normally go to guys with a 6+ ERA. He's not a 14 win player, but he'd probably replace guys who are huge negative win players. He'd effect days even when he isn't pitching. The bullpen would be in better order and they could use their better relievers for more of the meaningful relief innings instead of turning to the mop-up types . If he were to start, Cessa could stay in relief and some disaster would be kicked off the roster.
If Castillo makes 32 starts and the Reds can win 60% of them, that's 19 wins as opposed to maybe 9 or 10 wins if Cessa made those starts. Cessa in the bullpen would replace some huge negative. The WAR calculation doesn't capture pitchers very well. It only takes one bad inning to lose a game, but that one bad inning wouldn't count as a negative 1 WAR. Reduce the number of bad innings, reduce the number of losses.