BOSTON -- The Red Sox made official what was agreed to nearly a week ago, signing first baseman Mike Napoli on Thursday to a two-year, $32 million contract through the 2015 season. 

To make room on the 40-man roster, outfielder Alex Castellanos was designated for assignment.

After losing two starting players from their World Series-winning team early last week, the Red Sox retained a key piece by signing Napoli. The deal, first agreed to last Friday, became official Thursday after Napoli passed his physical.

Just moments after the first media report surfaced last Friday that Napoli was coming back to Boston, the slugger confirmed the news himself on Twitter.

"The beard is coming back to Boston!!! Couldn't be happier!!" Napoli wrote.

It certainly comes as welcome news to Red Sox fans and players, considering the events early last week, which included Jacoby Ellsbury agreeing to a seven-year, $153 million deal to go to the Yankees and Jarrod Saltalamacchia going to the Marlins after A.J. Pierzynski signed a one-year deal to take his place in Boston.

Aside from David Ortiz, Napoli is Boston's best power hitter.

In 139 games with the Red Sox in 2013, Napoli hit .259 with 23 homers and 92 RBIs while breaking a team record with 187 strikeouts.

However, Napoli's strikeouts weren't a big issue, considering the upside of his power, on-base skills and defense.

Napoli came up big for Boston a few times in October, most notably in the American League Championship Series, when he hit clutch home runs to fuel hard-fought victories in Games 3 and 5 against the Tigers.

Though he will always be known for his bat, Napoli adapted seamlessly to first base after converting from catcher.

Last December, the Red Sox originally agreed to terms with Napoli on a three-year, $39 million contract. But after spotting a degenerative hip condition during his physical, the deal was reduced to an incentive-laden one-year deal that wound up paying Napoli $13 million.

In other words, combine Napoli's 2013 earnings with what he will make over the next two seasons, and he will wind up getting $45 million from the Red Sox between 2013-15.