The Donovan McNabb era with the Washington Redskins was one long bout of buyer’s remorse.

What made McNabb and Coach Mike Shanahan such a poor match? The half-buried causes and resentments aren’t clear even to the participants, but let’s start with unreasonably high expectations that came with McNabb’s “stature,” as his agent puts it.

Maybe McNabb was done in by his own McNabbness.

The Redskins and McNabb seemed to expect something better than what they actually got. When the Redskins obtained McNabb from the Philadelphia Eagles in exchange for second- and fourth-round draft picks last summer, they thought they were getting a franchise headliner with deep experience, appearances in six Pro Bowls and five NFC championship games. Nothing wrong with that assumption — except they were also getting a 33-year-old veteran who after 11 seasons didn’t quite have the legs he used to, and worse, didn’t have the flexibility to adapt to a new philosophy, either. And who wasn’t inclined to admit, much less correct, some bad habits.

As for what McNabb thought he was getting in the Redskins, apparently he thought he would get a comfortable sinecure and unqualified support. He fell for the hype that the Redskins were just a couple of players away from contending. Instead, he got a team with pressing needs at a dozen positions, including a frail offensive line behind which he had to fight for his reputation. He got a new head coach in Shanahan who was trying to manage a thin roster while making his own transition to a new organization after 14 years with the Denver Broncos, and who knows how to go about things in only one way: his own.

As if that wasn’t enough, there was a subtle but plaguing complication. McNabb no doubt thought his experience could — and should — trump the imaginative flights of the Redskins’ young offensive coordinator, Kyle Shanahan. The offensive coordinator is the coach’s kid, who at 31 is younger than McNabb. When it became clear that Kyle’s precise, almost-computer-like scheme didn’t suit McNabb, who was going to defer to whom? Was McNabb really going to let a junior executive such as Kyle correct his reads, or his footwork, or his inaccuracy?

Only the Shanahans and McNabb know what really happened to sour their relationship — whether McNabb was just a scapegoat or whether he indeed was more image-conscious than industrious.

The Shanahans were criticized for their impolitic handling of his benching. But it also seems true that they put up with some complacency if not passive-aggressiveness from their quarterback and his agent, who continually intruded with talk about McNabb’s “stature,” as if that made him inviolable. McNabb’s diva quality is seldom remarked on, but it’s there.

As much as anything, the fraying of the relationship may have boiled down to a difference over basic demeanor. McNabb is renowned for his lack of urgency. He runs the offense at the same pace he knots his necktie. That is part of his composure. But it can also seem complacent and almost slow-pokey, and it was distinctly counter to the high energy, fast action and work obsessiveness demanded by the Shanahans.

When McNabb considers what he may want to do differently, he may want to reappraise his pace. The Shanahans are not the first to find fault with it. Of all the criticisms of McNabb last season, one that sticks as accurate came from ESPN analyst Tim Hasselbeck, who was with the Philadelphia Eagles for part of McNabb’s heyday, and who found some of the Shanahans’ complaints similar to those of Eagles Coach Andy Reid.

In a radio interview with “Mike and Mike in the Morning,” Hasselbeck said: “One of the things that drove them crazy in Philadelphia was the lack of tempo at which he practiced. . . . It was always something where you’re leaving the quarterback meeting and it would be, ‘Hey, listen, the head man wants a little more tempo today.’ Nearly every single day. That’s been the deal with Donovan McNabb. I know exactly what Mike Shanahan is talking about.”

It’s possible that McNabb’s slow tempo, his habit of ambling to the line of scrimmage, seemed perfunctory to the Shanahans. Maybe they thought McNabb was less than all-in. It may have planted the first seeds of distrust. If so, the impression was reinforced by his neutral play. Eventually, it was necessary to find out whether McNabb was holding back the offense, or the offense was holding back McNabb.

The verdict now seems clear, though the method of arriving at it by benching him was unpleasant and painful to watch. In the end, there was no arguing with the statistics: McNabb had the second-worst completion percentage of his career, and threw more interceptions (15) than touchdowns (14). It was obvious that the Shanahans had to look for a quarterback with whom they are more compatible.

That they’ve identified John Beck as that quarterback is an oblique comment on just how dissatisfied they were with McNabb. Among Beck’s qualities: he’s young, malleable, fleet, accurate, gung ho, and an apple polisher. Also, he’s unsung. Which apparently will be a relief to them.