Lefty Johan Santana would give the Red Sox the best rotation in baseball.

AP / Steven Senne

BOSTON — As recently as a week ago, the Red Sox seemed set to embark on a relatively tranquil offseason.

After re-signing free agents Curt Schilling and Mike Lowell, the Sox’ winter shopping list seemed short. The club needed to find an experienced fourth outfielder (or re-sign Bobby Kielty); obtain a corner infielder to replace Eric Hinske, and upgrade the backup catcher position.

That was before left-hander Johan Santana entered the picture. Days before the Winter Meetings get under way in Nashville on Monday, the Sox and Minnesota Twins are already engaged in talks for Santana, arguably the best pitcher in baseball.

Santana’s pending free agency — his current contract runs out after the 2008 season — and contractual demands — he is insistent on being the highest-paid pitcher in the game — have forced the Twins into sell mode.

The Red Sox, of course, are just one of a half-dozen or so interested suitors. The New York Yankees, New York Mets, Los Angeles Dodgers and Los Angeles Angels of Anaheim all have expressed their interest to the Twins, and some, like the Red Sox, have begun talks with new Twins GM Bill Smith.

A two-time Cy Young Award winner, Santana, 28, will not come cheaply — literally and figuratively. The Twins are seeking a minimum of three top prospects and/or young major-leaguers in exchange.

The St. Paul Pioneer-Press yesterday reported that the Sox had vaulted into the driver’s seat for Santana, with an offer of outfielder Coco Crisp, lefty Jon Lester, minor-league pitcher Justin Masterson and minor-league infielder Jed Lowrie.

Minor-league pitching prospect Michael Bowden’s name has also been discussed.

A source with knowledge of the negotiations last night confirmed only that the teams had exchanged multiple proposals this week and said “some positive momentum” had been created by the two clubs.

Last night, however, a deal was far from imminent.

Even if a team can satisfy the Twins, Santana’s contractual demands must also be met. Sources with direct knowledge of Santana’s deal say that the pitcher will not waive his full no-trade clause until he reaches agreement on a five- or six-year deal that will pay him well in excess of $20 million annually.

Until recently, it had been widely speculated that the Twins were eying a package that would include the Sox’ top two prospects: pitcher Clay Buchholz and outfielder Jacoby Ellsbury.

The Red Sox were adamant in their refusal to part with both, though they may be persuaded to include one of the two in a grouping that would probably also include Lester and others.

In addition to replacing Santana, 93-44 lifetime, in the rotation, the Twins also have needs in center field (free agent Torii Hunter departed for a five-year, $90-million deal with the Angels last week) and at third base.

Crisp could offer the Twins a potential replacement for Hunter. Crisp is under control for the next two seasons with salaries of $4.75 million in 2008 and $5.75 million in 2009, with a club option for $8 million in 2010. Even for the budget-conscious Twins, that contract makes Crisp eminently affordable.

Lester would join Francisco Liriano (coming back from Tommy John surgery) to give the Twins two promising lefties at the front of the rotation, while Masterson could, in another season, supplement a rotation that will likely feature Boof Bonzer, J.D. Durbin and Kevin Slowey.

Lowrie, an All-American at Stanford who reached Pawtucket in the second half of last season, could play any one of three infield positions for the Twins: second, short or third base.

Obtaining Santana would give the Red Sox the pre-eminent starting rotation in the game. Adding Santana to 20-game winner Josh Beckett and rising star Daisuke Matsuzaka could set the Sox up as prohibitive favorites for the next several years.

The cost of such a deal, however, would send the Sox’ payroll into the stratosphere. Even allowing for the fact that Santana is due $13.5 million in 2008, the last year of his current deal, the Sox’ starting rotation alone would be on the books for $43 million and their overall payroll would stand at $126.75 million — before the Sox determined the 2008 salaries of Dustin Pedroia, Kevin Youkilis, Jonathan Papelbon, Manny Delcarmen and a number of reserve players.

If the Sox should strike out on Santana, they might pursue Oakland’s Dan Haren, also said to be available. Haren isn’t left-handed and lacks Santana’s credentials, but he would be far cheaper in terms of salary; with just three years of major-league service time, he’s signed through 2010 for a total of $16.25 million, or less than $3 million more than Santana will earn next season.

At the very least, the Sox hope to succeed in driving up the asking price for the Yankees, who are more desperate to land a certified No. 1 starter, having been eliminated in the first round of the playoffs in each of the last three seasons.

smcadam@projo.com