Through a 5-0 start distinguished by notable wins at New England and over otherwise-undefeated Philadelphia, Chiefs quarterback Alex Smith was alert, aggressive and eerily accurate all over the field.

Playing with what coaches like to call controlled reckless abandon, he was a frontrunner for NFL most valuable player.

Even through seven weeks, he had completed 72.4 percent of his passes and averaged 283 yards a game, with 15 touchdown passes and no interceptions.

So, sorry, you can’t do all that and, presto, just be a bad quarterback, even if his numbers (four touchdown passes, four interceptions) and presence have dropped the last four games and drooped in the last two clunkers against the New York Giants and Buffalo.

What’s happened is more complicated than the simplistic idea that Smith has become spontaneously incapable of doing the exact things he was excelling at just weeks ago against some of the best competition in the league.

And the explanation helps account for why the intensifying clamor for replacing him with rookie and presumed future star Patrick Mahomes will, and should, go unheeded unless and until the Chiefs tumble out of playoff contention or Smith is determined to be hurt.

“That’s a tough position in the NFL; there’s a lot to learn, a lot that goes on, a lot that’s involved in that,” Chiefs offensive coordinator Matt Nagy told The Star. “So to be able to take somebody and just put them in there, you better be careful with it and you better have a good plan.

“And that’s not where we are right now. Alex is our guy, and we’re rolling, and we fully believe that we’re going to get right back on track and put this thing behind us. …

“There’s not one person on this team or staff that feels like Alex Smith is the problem.”

That may be true to the best of Nagy’s knowledge, though it may or may not be overall considering such observable phenomena as tight end Travis Kelce’s exaggerated-exasperated body language on the interception that snuffed out hopes of a late rally in the 16-10 loss to Buffalo.

But this isn’t just stubbornness or stay-the-course stuff from Nagy, who isn’t just trying to cover for Smith.

It’s best-chance-to-win-now stuff, because Mahomes just isn’t ready to run this intricate offense in anything other than a desperation sense … and Smith has it well within him to make this crystalize again if the rest of the parts are in good working order.

That’s really the crux of the matter — and something the Chiefs might not be able to get past.

Smith can thrive with functionality around him, with receivers being good-hands people and running dependable routes, with a sound running game and reliable protection. But he’s not exceptional enough to overcome the lapses the Chiefs are suffering in all those areas.

Consequently, with issues all around him, he looks like an entirely different player, so much so that it’s been easy to wonder if his arm is injured or he’s mentally fatigued from the punishment he’s been absorbing for weeks.

Maybe something along those lines actually is a factor, though Smith insists he feels as well as can be expected this time of year and jokes that some of the hits “that look the worst aren’t always.”

Maybe in some ways that’s true about how some of the games that look the worst aren’t, either.

Case in point: Smith was 23 of 36 for 199 yards with a touchdown and an interception last week against Buffalo.

He appeared jumpy in the pocket, threw a handful of passes poorly and seemed not to see several open receivers … or Tyreek Hill and Albert Wilson standing in the same place for a screen pass.

But the dynamics were off from the start — the Chiefs had little dialed up to go downfield and couldn’t generate a running game, for starters.

And Smith also had a couple passes dropped, receivers as noted ridiculously standing in the same place and several times appeared to be the wrongdoer when he threw where a receiver was supposed to be but wasn’t — apparently including on the late interception intended for Hill.

That sort of miscommunication on the key play, Chiefs coach Andy Reid said this week, “shouldn’t happen.”

Meanwhile, inconsistent protection has Smith on the move seemingly sooner than he needs to be at times with less field awareness in front of him than he’d had earlier this season.

You can call it paranoia if you want, but remember that it isn’t paranoia if they’re really out to get you … or about to get you.

It’s easy to criticize that from a distance, and some criticism surely is deserved — especially since few plays go as rehearsed and the margin between winning and losing often depends on the adaptation.

But it’s also fascinating that so many observers can be so sure they understand what Smith’s pass-reading progressions are supposed to be … or what he should be able to do in milliseconds depending on 21 moving parts on the field and objects instantaneously becoming closer than they appear.

“It’s a little bit different when you’re down there and you’ve got different people all around you and different things going on,” Nagy said, smiling.

(It also bears mentioning that Smith undoubtedly has a better feel for all of this after more than a decade in the NFL than would Mahomes, who perhaps could be more elusive than Smith, but isn’t as fast.)

With all that stems a cycle: less trust, more uncertainty, faulty decisions and even pressing, as Nagy suspects Smith has begun doing.

“It’s human nature, especially at that position, because you get all the glory when you’re winning and it all comes down on you when you’re losing,” he said. “And everybody wants the next guy.”

Understandably enough, actually.

And none of this is to say that Smith shouldn’t be accountable for his diminished play or that the Chiefs shouldn’t be thinking about at what point it’s appropriate to insert Mahomes for seasoning.

But even if they lose Sunday to the Jets, unless it becomes evident Smith is injured, it’s hard to envision the change coming with a playoff berth essentially on the line in two division games immediately following.

Because barring injury, he’s the same guy who showed that at his best he can make this engine go.

And with some help from his supporting cast, he’s the Chiefs’ best chance in the present before they turn over the future to Mahomes.

Vahe Gregorian is a columnist with The Kansas City Star/TNS.