The world champion Red Sox aren’t getting fat.

According to a person who knows Carlos Beltran, the switch-hitting right fielder has drawn strong interest from the Yankees’ blood rivals.

The person said the Yankees and Orioles are in on Beltran, too, but the Red Sox have been more aggressive.

Of course, it’s early in the process and that could change. Beltran, 37 in late April, is expected to turn down the Cardinals’ $14.1 million qualifying offer Monday. Teams signing him will forfeit a draft pick in June.

Monday is also the day the Yankees will learn whether Curtis Granderson accepts their qualifying offer. If he does it’s not likely the Yankees will have room for Beltran in an outfield that already houses Alfonso Soriano in left and Brett Gardner in center. If Granderson stays, he would play right field. Beltran is trending toward being a designated hitter, but he will command the type of money rarely given to a full-time DH not named David Ortiz.

Boston’s aggressive approach could stem from the possibility they could lose center fielder and leadoff hitter Jacoby Ellsbury as a free agent. If that’s the case, Shane Victorino could shift from right to center, or Jackie Bradley, Jr. could take over for Ellsbury with Victorino and Beltran playing the corners. One thing is for sure, Ortiz has a choke hold on the DH duties.

The Orioles are set in right field with Nick Markakis, but could be looking at Beltran as a DH or left fielder.

Beltran, an eight-time All-Star who played six-plus seasons for the Mets, is coming off a season in which he batted .296 with 24 homers, 84 RBIs and an .830 OPS. This past season was the second of a two-year, $26 million deal. He was an All-Star in each of his two seasons in St. Louis.

A career .333 (60-for-180) postseason hitter with 16 homers, 41 RBIs and a 1.1280 OPS, Beltran hit .268 (15-for-56) with two homers and 15 RBIs in 17 October games for the NL champions. Despite suffering bruised ribs robbing Ortiz of a grand slam in Game 1 of the World Series, he still batted .294 (5-for-17) with three RBIs in six Series games.