Arm in a sling, Bears QB Mitch Trubisky remains the enigma he’s always been

Sunday was supposed to be the measuring-stick game — the time to see how Mitch Trubisky fared against a defense that had seen him more than any other team in the NFL. It was different than his first three starts, when Trubisky played the Packers after sitting out the entire preseason, faced his former defensive coordinator with the Broncos and matched up against one of the league’s worst units in the Redskins.

The test lasted six whole plays.

That’s when Trubisky spun out of the huddle, rolled left, fumbled and was sacked by Vikings defensive end Danielle Hunter. He went to the injury tent, and then the locker room, and returned in the second half with a sling on his left arm and the diagnosis of a shoulder injury.

“I just — I want the kid to do really well,” coach Matt Nagy said. “How many plays was it, six? I’m sure that part is frustrating.”

Nagy wouldn’t specify the severity of the injury — despite being asked whether the shoulder was dislocated or broken — but said he didn’t believe the injury was season-ending.

It might not matter. The Bears are running out of games in which to evaluate Trubisky. Whenever he returns, he might not be at full strength — if the shoulder were dislocated, he’d wear a harness for the rest of the season.

The fundamental question — is Trubisky good? — might not be answered all season long.

That matters for reasons that go beyond the Bears’ won-loss record. Trubisky is eligible for a contract extension this upcoming offseason. The Bears could push that decision back a year, given they hold a fifth-year option that will keep him in Chicago through 2021. Still, Trubisky’s agents figures to push for an extension this offseason, the way they did for the Eagles’ Carson Wentz and the Rams’ Jared Goff — who have been in the league one year longer — earlier this year.

Then there’s this uncomfortable fact: playing without the Bears’

No. 2 receiver and behind a third-string guard, Chase Daniel ran the Bears’ offense well. He fared better than Trubisky did against the Packers or Broncos.

Trubisky remains an enigma — though Nagy said he was on the upswing.

“That part is frustrating because [Trubisky] had a really good week of practice, and every week he’s so excited to get back at it and really get this thing going,” Nagy said. “And so it’s — it happens early on, and now here he is, he’s out. . . . He’s out of the game and now he knows that we’ve got to just see where it’s at.”

The injury looked familiar to receiver Anthony Miller, who dislocated his shoulder last season.

“When I found out he had that shoulder, I was thinking to myself, ‘That’s a nagging injury,’ ” Miller said. “He’s going to have to do a lot of rehab on it, keep it tight, just do a lot of things for it. I think he’ll be fine.”

Miller wore a harness last year to keep his shoulder in socket.

“It kind of feels like you’re, not playing in a box,” he said. “But you’re being cautious of your movements so it doesn’t slip out [of socket] again. . . . You kind of learn how to play when you have this injury.”

It’s possible, then, that Trubisky doesn’t feel like himself the rest of the season.

Then what do the Bears do? What do they have?

They were supposed to find out Sunday.

“They have a top defense, and I know Mitch embraces the challenge and he loves the challenge . . .” cornerback Prince Amukamara said. “I was definitely looking forward to it. But I was excited how Chase performed.”