Thanks to all who responded, I came to the independent conclusion to keep Rivera shortly after my initial post. I did some rough calculations in my head along these lines...a true superstar/mvp type is worth around 10 games over replacement level over an entire season. Thus Mariano at best is worth around one win per 16 games. So I'm losing about a win due to Mariano's absence for 14 games, assuming that all of his innings go to replacement level pitchers. While most of those innings will, at least some will go to mediocre instead of lousy pitchers. So one win at most is lost.
But then I had to figure for the rest of the 34 games for the season I'm regaining at least parts of that "lost win" if I keep Rivera; whatever marginal difference there is betweeen Rivera and Devine over 33 games, which I appoximate as (10 season wins - 5)/162*34 =1+ wins. So in a way this approximates Marcus' calculations using Strat's pricing system, and it comes out to be a wash. And therefore having Rivera for the playoffs potentially becomes the thumb on the scale to resolve the problem.
Rivera is critical to my team because I dont use him as a one inning closer but try to squeeze a rate of one inning/game out of him over the course of the season. That being said, I'd use Devine the same way, so again I gotta keep Rivera after that balances out.