Delaplane has been the best reliever in the Mariners minor league system in the last few years. In his 3 years in the Mariners system, he has made 100 relief appearances and has a 2.29 ERA. He did not pitch in a game last season but he was a member of the Mariners player pool.
In 2019 the highest level he reached was AA where he had a 0.49 ERA in 37 innings pitched. While his minor league stats were incredible and he was a part of the Mariners summer camp, he was never added to the 40 man roster and never got an opportunity to debut.
2021 could be Delaplanes time to shine, however, with all of the turnover in the Seattle bullpen Delaplane will almost certainly debut in 2021, the question is could it be on opening day? If Delaplane has a good spring training against MLB competition he could do enough to break into the Mariners 7 man bullpen that only has a few players who are certainly going to be on the opening day roster.
My prediction is he does not make the opening day roster, but after a solid spring training and a month or two of solid pitching in AAA, once there is an injury in the bullpen Delaplane is the first reliever called up.