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Seattle RHP Luis Castillo (4-4, 3.29 ERA) will start for the Mariners on Wednesday.
Red Sox RHP Walker Buehler (5-4, 5.01 ERA) starts against RHP Bryan Woo (5-4, 3.39) in the middle game of the series Tuesday.
Seattle will now have to try to win points off Group B powers Atlético Madrid and Paris Saint-Germain to avoid group stage elimination.
Seattle right-hander Logan Gilbert (1-1, 2.37) will return from the injured list and start for the Mariners on Monday against the Red Sox.
The D-backs send LHP Eduardo Rodriguez (1-3, 6.70 ERA) to the mound on Wednesday while the Mariners counter with RHP Bryan Woo (3-5, 3.07).
Mariners RHP Bryan Woo (5-3, 3.07 ERA) starts Tuesday against Diamondbacks RHP Brandon Pfaadt (7-4, 5.51).
Stefan Frei had two saves for the Sounders.
Emerson Hancock (2-2, 5.19) pitches for the Mariners at Arizona.
Baltimore RHP Zach Eflin (4-2, 4.46 ERA) faces RHP Bryan Woo (5-2, 2.82) in the series finale Thursday afternoon.
Seattle right-hander Emerson Hancock (2-2, 5.64 ERA) starts Wednesday.