ANAHEIM — Only a few weeks ago, at the mid-summer All-Star break, the field for the American League postseason looked assured.

The Red Sox comfortably led the American League East, the Los Angeles Angels of Anaheim were safely ahead in the West, and the Detroit Tigers were battling the Cleveland Indians for the A.L. Central title, with the second-place finisher the presumptive wild-card entry.

A month later, the playoff landscape has been significantly overhauled. The Red Sox lead — 6½ games before their series opener with the Angels here last night — has been compromised some, but not truly threatened. Boston’s lead is still the largest of any division leader and with 51 games left, they would need only play one game over .500 (26-25) the remainder of the way to reach 94 wins, probably enough to qualify for the playoffs.

But the rest of the field remains in flux.

Both the Indians and Tigers have been in a free-fall. Before last night, the Tigers had dropped nine of their previous 10, including the last five in a row, allowing the Indians to overtake them for the division lead. The Indians hadn’t reached first place in the Central on merit; they got there by virtue of being not quite as cold as the Tigers, losing seven of their last 10.

While the Indians and Tigers foundered, the Twins began their second-half resuscitation, taking two of three from Cleveland to move within 4½ games of the division lead.

“We smell blood right now,” crowed Twins center fielder Torii Hunter. “We’re ready to go.”

In the West, the Mariners continue to cling to the Angels, entering last night just three games off the pace for the division lead, a position few thought imaginable a month or so ago.

Finally — and most critically, if you’re a Red Sox fan — there are the resurgent Yankees, who won their fourth in a row yesterday even before the Red Sox arrived at Angel Stadium.

The Yankees have scored runs at a breathtaking clip for the last month, battering opponents into submission, and their lineup is about to get stronger still with the return of Jason Giambi.

In early July, the Sox seemed certain to be matched with either the Tigers or Indians — whichever team didn’t claim the Central crown. Now, their list of potential ALDS opponents seems to expand almost daily.

With the West up for grabs, either the Mariners or Angels could be in line, particularly if the Yankees emerge as the wild-card entrant. Rules preclude teams from the same division meeting in the Division Series, so should the Yanks nail down the wildcard, the Sox would meet the division champion with the lesser record.

Assuming the Sox hold off the Yankees and win the East for the first time since 1995, they could meet any one of five teams in the first week of October: Cleveland, Detroit, Minnesota, Los Angeles of Anaheim and Seattle.

Each opponent would represent different challenges. The Sox have never played well in the Minneapolis Metrodome and their recent troubles at Safeco Field are well-documented. The Twins and Mariners also boast deep bullpens, an asset made more significant come playoff time.

Cleveland and Detroit feature deep starting pitching. In Fausto Carmona and C.C. Sabathia, the Indians finally have two true No. 1 starters, something their World Series teams of 1995 and 1997 had.

Finally, there are the Angels, who play a relentless, aggressive style seldom seen anymore — especially in the American League — and a matchup that could present problems for the Sox.

As the owners of the best record in the game, of course, the Red Sox have the luxury of focusing merely on their own play. As Josh Beckett pointed out Sunday, if the Sox keep winning series, they won’t have to worry about the Yankees — or anybody else, for that matter.

Bur before the Sox can solidify their own spot in the October tournament, the field of possible opponents will continue to rearrange itself on an almost daily basis.

smcadam@projo.com