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Frei was alert as he left the field, Sounders coach Brian Schmetzer said.
Right-hander Logan Gilbert (2-2, 3.40) is scheduled to start Tuesday at the New York Yankees.
Seattle RHP Logan Gilbert (2-2, 3.55) and Kansas City Noah Cameron (2-4, 2.79) start Wednesday in the third game of the four-game set.
Royals RHP Michael Lorenzen (4-8, 4.91 ERA) starts Tuesday against RHP Emerson Hancock (3-3, 5.43) in the second game of the four-game series.
The Mariners return from their second 10-day road trip in a month to face Kansas City on Monday.
Seattle hosts Columbus on Sunday.
Seattle starts a three-game series at Texas on Friday.
Seattle RHP Emerson Hancock (3-3, 5.43) and Minnesota RHP Simeon Woods Richardson (2-4, 5.06) start Thursday’s finale of the four-game series.
The Fever return home to face the Los Angeles Sparks on Thursday, while the Storm will host Connecticut on Friday.
Twins RHP Joe Ryan (7-3, 3.06) will face Seattle RHP George Kirby (1-3, 6.16) on Wednesday night.