The Boston Red Sox and Detroit Tigers appear to be the front-runners for the services of Cuban outfielder Rusney Castillo, two league sources said Thursday, with a decision expected no later than the weekend.

The Red Sox would like to add Castillo to an outfield that already includes another Cuban defector, Yoenis Cespedes. The Tigers, who have fallen behind the first-place Kansas City Royals in the AL Central, are looking for a replacement for center fielder Austin Jackson, who was traded to the Seattle Mariners as part of the three-way deal in which Detroit acquired left-handed ace David Price from the Tampa Bay Rays last month.

The Red Sox and Tigers lead the list of teams vying to add Cuban outfielder Rusney Castillo. A decision is expected no later than this weekend. RAUL ARBOLEDA/AFP/Getty Images

A source with direct knowledge of the negotiations said the Red Sox are "optimistic" they will ultimately sign Castillo.

The Tigers are 10-10 since that trade, with Price losing 1-0 to the Rays on Thursday despite allowing just one hit and no earned runs. Manager Brad Ausmus has been using Rajai Davis and Ezequiel Carrera in center field in Jackson's place.

Castillo is viewed as being close enough to the big leagues that a contender that signs him this month could have realistic visions of having him on its roster this October.

The Tigers have been to the World Series twice in the previous eight years but lost both times, first to the Cardinals in 2006, then to the Giants in 2012. They have not won a World Series in 30 years -- they beat the San Diego Padres in five games in 1984 -- and owner Mike Ilitch is 85 and in declining health, giving their quest even greater urgency.

Clubs involved in the bidding, according to ESPN.com's Jayson Stark, believe Castillo ultimately will sign a five-year contract in the range of $50 million to $60 million. That would rank as the second-largest deal ever signed by an amateur player from Cuba or any other country, behind only Jose Abreu's six-year, $68 million contract with the Chicago White Sox.

Castillo, 27, has attracted the interest of a number of teams since his defection from Cuba last December. He established residency in Haiti last spring, and he held a public workout in Florida last month that was attended by 28 of the 30 major league clubs.

At 5-foot-9, 185 pounds, Castillo isn't a pure masher in the mold of Abreu or Cespedes. But his power/speed combo has drawn comparisons to Andrew McCutchen and Yasiel Puig.

Although he played the infield at times in Cuba and took ground balls at shortstop during his workout, his best position is center field, and that's where the teams pursuing him are likely to play him. Because he is older than 23 and played five seasons professionally in Cuba, he is exempt from international signing-bonus pools. Castillo is represented by Jay Z's Roc Nation Sports.