A baseball source said the radial collateral ligament was found by Dr. Donald Sheridan to be “95 percent torn.’’ The surgery went well and Lowell is scheduled to return to his home in Florida today.

The injury occurred during the final days of the regular season, and the third baseman was cared for by the Sox medical staff. But the severity of the tear was not discovered until after an agreement was in place to trade Lowell to the Rangers and he underwent a physical 10 days ago.

The Sox were prepared to send Lowell and $9 million to the Rangers in return for Triple A catcher Max Ramirez. The Rangers would have been responsible for $3 million of Lowell’s salary.

The Sox are expected to revisit the idea of trading Lowell, with the Rangers still a possibility. But he will first have to prove himself healthy. The prescribed rehabilitation period of 6-8 weeks would put Lowell on schedule to start spring training with his teammates. Pitchers, catchers, and injured players are scheduled to report to Fort Myers, Fla., Feb. 18. The first full-squad workout is Feb. 24.

Prior to the trade being called off, Sox general manager Theo Epstein said he was prepared to start the season with Kevin Youkilis moving to third base and Casey Kotchman penciled in to play first.

The return of Lowell to the roster may not change those plans. In a question-and-answer session on NESN.com a few days ago, manager Terry Francona wrote that Lowell would be a candidate to get at-bats as a designated hitter and could play “a little’’ first base.

Lowell, who turns 36 in February, hit .290 last season with 17 home runs and 75 RBIs after undergoing offseason surgery on his right hip.

Peter Abraham can be reached at pabraham@globe.com.