They recalled pitcher Art Warren from Triple-A in a corresponding move.
This is a concerning setback for the 31-year-old, who has been blighted by injuries in the past and underwent surgery on an elbow issue in September 2019. He also started the 2021 campaign on the injured list because of back concerns.
Gray suffered a forearm injury and trapezius strain in 2016 and struggled from a statistical perspective with a 5.69 ERA after a spectacular 2015 effort.
He made the All-Star Game and finished third in the Cy Young voting in 2015 with a 2.73 ERA, 1.08 WHIP and 169 strikeouts in 208 innings. He returned to solid form again in 2017 with a 3.55 ERA for the Oakland Athletics and New York Yankees but struggled in 2018 on the way to a 4.90 ERA.
Cincinnati acquired him via trade prior to the 2019 campaign in the hope he would resemble the 2015 version of himself from Oakland. He responded by making the All-Star Game in his first year in the National League with a sparkling 2.87 ERA and was solid in 2020 with a 3.70 ERA.
He has followed with a 3.42 ERA, 1.30 WHIP and 65 strikeouts in 50 innings in 2021.
The Reds can still rely on the combination of Luis Castillo, Tyler Mahle and Wade Miley in the rotation while Gray is sidelined, but they could use him back and healthy if they are going to challenge for a playoff spot.