During his three full seasons as head coach of the Dallas Mavericks, Avery Johnson compiled one of the most successful regular season records of any coach in basketball. But his dismissal today comes as absolutely no surprise.

Johnson’s tenure with the team was thought to be on shaky ground going into this season. The Mavericks had suffered two consecutive playoff disappointments. In 2006, they were minutes away from a stranglehold on their first NBA title, but they coughed up that game as part of losing four straight to Miami. The following season, after racing out to a league-best 67 wins, they were ousted in the first round of the playoffs by the Golden State Warriors in the biggest upset in postseason history.

This season, the Mavericks lost in the first round to the New Orleans Hornets. It wasn’t an upset, but it was a disappointment. After Tuesday’s loss, Johnson’s record was a study in contrasts. His teams went 194-70 in the regular season, but 23-24 in the playoffs.

The disparity between the two records points out a crucial aspect of Johnson. The Mavericks were his first coaching job. Mavs owner Mark Cuban asked a rookie to do a veteran’s job. Under Johnson’s predecessor, Don Nelson, the Mavericks were a run-and-gun team that played little defense. Johnson brought in a new philosophy and slowed the team down from one of the fastest in the league to one of the slowest. Remarkably, they lost little of their efficiency in the switch from a finesse game to a smashmouth offense. Johnson also led a dramatic improvement in the Dallas defense: In the year before he took over, Dallas ranked 26th in Defensive Efficiency (points allowed per 100 possessions). In his best season on the Dallas bench (2006-07), the Mavericks ranked fifth.

In theory, these changes should have made the Mavericks much more formidable in the postseason. Instead, this is where Johnson’s inexperience showed. During the regular season, you simply hone your team’s strengths as best you can, since it’s a nonstop 82-game marathon. In the postseason, where teams gets the same opponent for upwards of seven times over a two-week period, there is greater opportunity to engage in tactics and strategy. And this is where Johnson failed.

During the Miami series, Heat coach Pat Riley made two adjustments after Game 3. He changed the focal point of the offense from Shaquille O’Neal to Dwyane Wade, and he altered the defense to be a variation of a box and one zone, so that his team could double-team Dallas forward Dirk Nowitzki almost every time he got the ball. It worked: Miami won three straight and a title, while the Dallas coaching staff made few attempts to react to the Heat’s new strategy.

A year later against Golden State, the mistakes were even more glaring. The Warriors were a small, quick team, so the Mavs needed to press their height advantage and attack the rim. Instead, the Mavericks spent six games struggling to get their perimeter game untracked while Golden State, not a stellar defensive team, turned the vaunted Mavericks offense into a shadow of its regular-season self.

This season’s disappointment can’t be laid entirely at Johnson’s feet. The team dealt their excellent young point guard Devin Harris and two first-round draft picks to the Nets for Jason Kidd. The trade was done with the thought that the road to the NBA Finals went through Los Angeles. Kidd is an especially good defender on shooting guards such as Kobe Bryant. Unfortunately, the Dallas brain trust failed to consider where the road to Los Angeles might lead. The first stop was in New Orleans, where the Hornets are led by point guard Chris Paul, the kind of player that Kidd struggles to guard. The Mavericks lost the series in five games.

Following the Golden State debacle, the Mavericks hired as an assistant coach Paul Westphal, a former player and coach who in seven seasons with Phoenix and Seattle compiled a 257-159 record. He has to be considered the front-runner for the position. But Cuban, a dotcom millionaire, prides himself on thinking outside the box, so anything is possible. The Mavericks have only six players under contract for next season, and one of them (Kidd) is in the final year of his deal, which could make him trade bait. The new coach in Dallas will certainly have a different supporting cast for his team and he could have a new nucleus. Meanwhile, Johnson’s regular season success means that his voice mailbox is probably already full. Lots of bad teams would like to improve their defense and suffer their disappointments in April and May, rather than February.

While Johnson was the first playoff coach shown the door, news of a second postseason firing is in the air. SI.com is reporting that Phoenix Suns coach Mike D’Antoni will be shown the door following his team’s first-round loss to San Antonio on Tuesday night. The report cites philosophical differences between D’Antoni and Suns general manager Steve Kerr. But Phoenix is in a different boat than Dallas. Whereas the Mavericks have some flexibility to reshape their roster, the Suns don’t. This may make the Phoenix job less attractive to prospective coaching candidates. The Suns have two ageing stars, O’Neal and point guard Steve Nash; an All-Star forward in his prime, Amare Stoudemire; two excellent role players, guards Raja Bell and Leandro Barbosa — and not much else. That’s a fine cast if the goal to make the playoffs, but the Phoenix mission is finals or bust. D’Antoni has been very successful in Phoenix (his record is 253-136 in the regular season and 26-25 in the playoffs). The urgency to win now will make filling D’Antoni’s shoes much, much harder. Burying the hatchet may be the best choice in a set of bad alternatives.

mjohnson@nysun.com