Portland coach Giovanni Savarese after Saturday night's game said a Minnesota United player directed a racial slur at Timbers star Diego Chara.
Savarese started his postgame media conference by speaking for nearly two minutes about what he called a "racist situation, a racial discriminatory word" said to Chara in the second half at Portland's Providence Park.
Savarese called the alleged incident "not acceptable" and said he was "very and extremely disappointed that still at this time we have to deal with situations that should not be happening in any sport or anywhere."
He did not name the Loons player nor specify the word or phrase used.
In a Sunday statement, Major League Soccer said it had already started an investigation. It said further information will be provided when the investigation is complete.
"MLS has zero tolerance for abusive and offensive language, and we take these allegations very seriously," the statement said.
Minnesota United released its own statement, saying it has been investigating the matter and that the unidentified player involved denied making "any derogatory remarks." A team spokesman said the player told coach Adrian Heath and several teammates he did not say such things and those teammates told Heath and others they didn't hear him say such things.
"MNUFC is built on inclusivity and respect and does not tolerate discrimination under any circumstances," the statement said.