will be sidelined from NBA games for the foreseeable future ... the league just slapped him with a huge suspension after it says he violated their anti-drug program.

According to the NBA, the Cleveland Cavaliers forward tested positive for banned substances ibutamoren and SARM LGD-4033 ... and he'll now be barred from playing in the team's next 25 games for it.

Both drugs are considered to be performance enhancers ... ones that help users improve their strength and speed up their recovery.

Thompson has yet to publicly address the positive tests.

The league said Tuesday that Thompson's suspension will begin on Wednesday ... when Cleveland takes on the Milwaukee Bucks.

The 32-year-old -- 's ex -- just joined the Cavs in September ... and he recently seemed fired up about the potential of this season -- after he switched his jersey number from 12 to 13, digits he wore when he first started in Cleveland back in 2011.

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"Wearing any other number didn't feel right," he said. "Back together, Number 13 forever."

In 36 games so far this season, Thompson is averaging 3.

8 points, 3.9 rebounds and 1.1 assists per contest.