The Mets signed Betances to a one-year deal with a player option for 2021 in December. The deal will guarantee him $10.5 million.
The New York native is 0-1 with a 6.10 ERA in 13 appearances this season. He is averaging 93.2 mph on his fastball, down from the 97.8 he averaged on the pitch in 2018, his last of five straight seasons with at least 100 strikeouts.
In 2018, Betances made 66 appearances, with a 2.70 ERA and 115 strikeouts in 66 2/3 innings. He was the first reliever in baseball history to have 100 or more strikeouts in five consecutive seasons.
Matz was moved to the bullpen and felt discomfort in a relief appearance on Saturday.
Matz is 0-4 with an 8.63 ERA in six appearances and Saturday was his first time in relief this season. He allowed four combined runs in his first two starts, but was tagged for 19 runs in his next three starts and pulled from the rotation after allowing six runs in a 6-2 loss at Philadelphia on Aug. 15.
Matz made his debut for the Mets in 2015, but he has spent time on the injured list in every season.