"I've been in the league way too long to have a mental lapse like that," McCollum told ESPN's Adrian Wojnarowski of leaving the bench during an altercation in a preseason game Wednesday.
"I want to apologize to my teammates and the organization for putting our team in this situation. The Western Conference is already tough enough as it is. It won't happen again."
Under NBA rules, McCollum must serve the suspension in the first game he is physically able to play in. Because he's currently not listed on Portland's injury report, he will miss the 2017-18 lid-lifter.
McCollum left the bench area after a shoving match broke out between teammate Caleb Swanigan and Suns center Alex Len. The NBA has strict rules about entering the floor unauthorized, but McCollum wouldn't make excuses.
"Lesson learned," he said. "I take full responsibility for those eight expensive and costly steps."