The New England Revolution say Lee Nguyen is "not honoring his contract" after the forward failed to appear for the first day of preseason training on Tuesday.
Nguyen has reportedly asked for a trade, but new Revolution coach Brad Friedel rejected the prospect of a deal when speaking to MLSsoccer.com last week.
And after Nguyen was a no-show in Foxborough on Tuesday, general manager Mike Burns told MLS's website: "Lee didn't report to preseason and he's not honoring his contract."
"We'll handle the matter internally, and we've already had discussions with MLS. It'll be handled appropriately. We have zero tolerance for guys who don't honor their contracts."
Nguyen, who made a salary of $500,000 last season, is entering the final guaranteed year of his contract.
The U.S. international has 51 goals and 49 assists in 191 MLS games.
Earlier on Tuesday, the Revs announced they had signed Ecuador attacker Cristian Penilla on loan from Pachuca. The deal includes an option to buy the 26-year-old in 2019.
New England said Penilla would sign as a designated player or using targeted allocation money, but did not confirm which option would be used.