WASHINGTON — The first mystery of the season has arrived — forward Bobby Portis’ bruised left rib.

The new Knick has no idea how he injured his ribs, but he felt it a few days ago and it cost him his preseason debut with the Knicks on Monday. Portis was unable to face his former team, the Wizards, and wore a wrap around his stomach in the locker room before tipoff.

“It’s been bothering me for a couple of days,’’ Portis said. “But this morning it really kind of acted up. I wanted to push through it but I couldn’t have played. I was going to try to play this morning, fight through everything, but around 1 p.m. I made that decision. And we just deal with it.”

It’s unclear if it happened when Portis rose for a dunk against Mitchell Robinson during Saturday’s open scrimmage and got blocked at the rim.

“I don’t even remember,’’ Portis said. “I just play hard. During war time, during basketball time, you don’t really feel it. You kind of feel it after everything is over.”

The Knicks lavished Portis with a two-year, $32 million deal in the offseason with the second year being a team option. The Wizards were blindsided, as Washington coach Scott Brooks had wanted him back before the Knicks went so high.

“I love him, love his spirit, love his energy, love his enthusiasm,’’ Brooks said. “It didn’t work out. He got a great contract. That’s a good contract and it’s a good [one] for the Knicks as well. He’s going to get better. He played well for us. He played big. He’s a tremendous shooter and finishes around the rim and rebounds pretty good.’’

With Marcus Marcus starting over Kevin Knox at small forward, the Knicks’ 2018 lottery pick said he’s fine with it but still feels he’s got a shot at the job.

Knox said his goal is to shoot 40 percent from the field.

“I think that’s one of the strong suits, this year is going to be the players coming off the bench,’’ said Knox, who scored 12 points on 5-for-12 shooting.

Coming off the bench for starter Elfrid Payton, point guard Frank Ntilikina ran the show with aplomb in nine first-half minutes. On his first possession, the maligned Frenchman bagged a 20-footer, then hit another mid-range shot. He was a plus-11 at the half, but his shooting faltered and he wound up 3 of 10 (1-for-4 from 3) with two assists. He finished with nine points, three assists and five rebounds in 23 minutes.

Wizards forward Davis Bertans could still be with the Spurs if Morris had realized early on he didn’t want to play with San Antonio. Morris reneged on his deal, but not before the Spurs traded Bertans to the Wizards. Spurs coach Gregg Popovich is irate he lost Bertans and never got Morris, who joined the Knicks instead.

“Of course I wasn’t happy to leave,’’ Bertans told The Post. “It wasn’t my choice as you know. I really enjoyed those three years. There’s no hard feelings. I understood what the team was doing. I still had talks with [former general manager and current Spurs CEO] R.C. [Buford] and Pop and ended on a good note. You never know what could happen in the future.’’

Indeed, the Spurs could always try to get Bertans back.