Sounders central defender Chad Marshall was taken from Starfire Sports Complex by ambulance Wednesday after falling during training.
"He fell, hit his head, we’re obviously taking all the precautions," coach Sigi Schmid said."… You’re always going to err on the side of being conservative in a situation like that – anytime it’s a head or a neck – and that’s what we’ve done, and we’ll know more later."
Local media reported that some Sounders said Marshall had movement of his extremities.
Marshall is the three-time and reigning MLS defensive player of the year. He joined the club in 2014 after 10 seasons with the Columbus Crew.
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The Sounders have an off-day from training Thursday. They will return Friday, preparing for their Sunday home match against the league-leading Los Angeles Galaxy.
Seattle already has lost starting central defender Roman Torres for the season following a knee injury at San Jose.
The Sounders’ top reserves at central defense are Brad Evans and Zach Scott, with second-year man Damion Lowe also available.