The NFL returned Thursday night with many of its trademark attributes: A close game, some debatable officiating calls and the specter of an undiagnosed concussion.

Some will remember the Denver Broncos' 21-20 victory over the Carolina Panthers for its thrilling conclusion: Panthers place-kicker Graham Gano was wide left on a field goal attempt with 4 seconds remaining.

Others will wonder why Panthers quarterback Cam Newton absorbed upward of five helmet-to-helmet hits, only one of which was penalized by referee Gene Steratore's crew. And many will question why Newton was never observed to have taken a concussion test, and whether his postgame condition will impact the NFL's new series of disciplinary measures for teams that violate the league's concussion protocol.

Let's take the latter issues one at a time.

First, it's important to remember that there are two standards for hits to the head. A player in a defenseless position, which for a quarterback means he is in the pocket, cannot be hit in the head or neck area by an opponent's helmet or face mask at any time, per Rule 12, Section 2, Article 7(b) of the NFL rule book. It's considered unnecessary roughness or roughing the passer, and it should have been called late in the second quarter on Broncos linebacker Von Miller after the play that started this entire sequence of events.

Newton was writhing in pain on the sideline afterward, but the Panthers announced it was because of a leg injury.

The other standard comes into play when the quarterback is outside of the pocket, as Newton often is. Helmet-to-helmet hits are legal in those situations, when the quarterback has become a runner, but officials still can call a penalty when a defender uses his helmet to butt, spear or ram an opponent violently or unnecessarily, as described in Rule 12, Section 2, Article 6(i). That's what Broncos linebacker Brandon Marshall did in the third quarter -- launching directly into Newton's helmet just after he threw the ball -- and was not penalized.

The only time a Broncos player was penalized for a hit on Newton came in the fourth quarter, when safety Darian Stewart speared the side of his helmet on a play that ultimately was wiped out by offsetting penalties.

Helmet-to-helmet hits are among the most difficult for officials to adjudicate in live action, and they are not reviewable. Clear contact in a slow-motion replay is not always obvious when it happens in full speed, and a whipping action of the head is not a fool-proof sign that illegal contact occurred.

But when viewed as a whole, what happened Thursday night ran counter to everything the NFL is attempting to achieve on player health and safety. An aggressive defense pummeled the league MVP with multiple hits to the head, resulting in only one offsetting penalty, leaving him battered and raising questions about his treatment.

The reason the NFL wants to avoid helmet-to-helmet hits, of course, is that they can cause concussions. And it was surprising, and frankly reminiscent of a similar instance with New England Patriots receiver Julian Edelman in Super Bowl XLIX, that Newton wasn't removed for a concussion test after several of the hits -- especially the one Stewart delivered in the fourth quarter.

The NFL has positioned unaffiliated neurotrauma consultants on each team's sideline for every game. It also has a certified athletic trainer (ATC) sitting in the press box to help watch for concussion symptoms. The ATC has the option to call a medical timeout during the game to ensure proper treatment, a rule put in place after Edelman continued playing in the Super Bowl two years ago after taking a massive hit and stumbling as he got up.

It's possible that Newton was tested between series, but he appeared to exhibit a classic concussion symptom after the Stewart hit and remained in the game as the Panthers tried to get into scoring position. On its list of potential concussion symptoms, the NFL's concussion protocol includes: "Slow to get up following a hit to the head."

Go back and watch Newton's reaction to Stewart's hit. He remained face down on the field for several moments before pulling himself up. It's possible Newton was simply exhausted, but a quick concussion test would have revealed whether that was in fact the case. (Update: The NFL said in a statement Friday morning that medical officials reviewed video of the ensuing game stoppage and determined there was no reason to conduct a test.) Newton told reporters that he was asked "a couple of questions" after the game to determine his condition, but was not placed in concussion protocol.

You're going to hear a lot more about these issues in the coming days. You can expect fines for Stewart and almost certainly Marshall as well. You also can expect to hear discussion about the NFL's new disciplinary system for violations of the concussion protocol, a document announced jointly by the league and the NFL Players Association over the summer. Fines and draft picks could be at stake.

Welcome back, NFL. We missed you.