Perhaps we have different criteria on what constitutes a "proven" reliever.
Approaching his 28th birthday in October, Carson Smith has barely one year of MLB experiences as the righthander recovers from Tommy John surgery. His work in 81 MLB innings taken as a whole is quite impressive although Smith was not without his struggles when he won, then 11 weeks later lost, the closer's role on a 2015 Seattle team that dropped 86 games. Smith relied heavily on his slider, which may or may not have contributed to his elbow injury.
The Mariners certainly traded high on Smith, who may develop into the hoped-for setup stud ... or not.
Ariel Miranda may been nothing more than a rich man's Roenis Elias, another Cuban lefthander who is less than one year older than Miranda. Last offseason Miranda was not a projected starter for the Mariners but with 25 starts this year has led a decimated Seattle staff that has used 37 pitchers, including 16 who have made starts. As a low-end starter with fly-ball tendencies, Miranda may be well-suited for Seattle with its superb defense covering an expansive outfield.
Let's hope the Red Sox get three productive years from Smith and the Mariners get five more productive years from Miranda ... but I have my doubts on each count.