What do you base this on?
A pitcher's value is based on how many outs he can get for his team. An average starter, accounting for wear and tear, can give you about 25 starts of 5.5 innings each, or roughly 135 innings or 405 outs. (Last season Michael Wacha started 23 games and threw for 127.1 innings, and Rich Hill 26 games for 124.1 innings. And they were no. 2 and no. 3 starters, respectively.)
A top reliever appears in a lot more games, but provides fewer innings of work. Last season, the Red Sox' top 2 relievers were John Schreiber (64 appearances, 65 innings) and Ryan Brasier (68 appearances, 62.1 innings). Garrett Whitlock threw for 78.1 innings, but it includes 9 starts. This season, in spite of his in-season injury and recent midseason ramp-up, Sale has given the Red Sox 77.2 innings already. That's more than any reliever on the team. (I don't count Nick Pivetta as a reliever. He's a starter used mostly after an opener.)
Sale's problem is that in the last few years he kept getting hurt. Moving him to the bullpen will certainly reduce his innings, but not the likeliness of him getting too hurt to pitch. And when he's good enough to pitch, you usually get 5+ innings out of him every 5 days, which is more than 1-2 innings every other day. The only scenario in which Chris Sale is converted to a reliever is if his stuff has regressed so much that he is unable to pitch for more than 3 innings without getting lit up. In that scenario he is a highly-fungible middle reliever on the last year of his contract.