As Philadelphia Phillies general manager Ruben Amaro Jr. explores trades involving first baseman Ryan Howard, he faces challenges beyond trying to move an underperforming player with a big contract.

Under the terms of Howard's five-year, $125 million contract with the Phillies, he can block trades to 20 teams. That leaves only nine clubs where Howard can be traded without his permission.

Ryan Howard had 23 home runs and 95 RBIs last season but led the majors in strikeouts and posted a career-low slugging percentage. AP Photo/Gene J. Puskar

A source familiar with Howard's contract told ESPN.com Monday that those nine teams are: the Detroit Tigers, Kansas City Royals, Los Angeles Angels, Seattle Mariners, New York Yankees, Texas Rangers, Tampa Bay Rays, Baltimore Orioles and Boston Red Sox.

Howard's list appears to be about strategy as much as personal preference. Most of the nine teams where Howard can be traded without his consent either have small payrolls or no need for a first baseman/DH, so the list doesn't appear to provide many promising options for Amaro.

According to a report by MASN's Roch Kubatko, the Phillies recently tried to package Howard with outfielder Marlon Byrd in trade talks with Baltimore, but the discussions went nowhere. The Phillies subsequently dealt Byrd to Cincinnati for minor league pitcher Ben Lively.

Howard, the National League Rookie of the Year in 2005 and the league's MVP the following season, led the majors with 262 home runs from 2006 to '11. But he missed a total of 173 games with injuries in 2012 and '13, and he continued to show signs of decline last season, hitting .223 with 23 home runs, 95 RBIs, a .310 on-base percentage and 190 strikeouts in 153 games.

The Phillies have made no secret of their desire to trade Howard, who turned 35 in November and will earn $25 million in both 2015 and 2016. The Phillies have a $23 million club option for 2017 with a $10 million buyout, so Howard is guaranteed a minimum of $60 million through the end of his contract.

Amaro already has traded Byrd and fellow veterans Jimmy Rollins and Antonio Bastardo this offseason in response to the Phillies' 73-89 record and last-place finish in the NL East.

During a recent radio appearance on 97.5 The Fanatic in Philadelphia, Amaro indicated that a trade would probably be in the best interests of both Howard and the Phillies.

"We've talked to Ryan,'' Amaro told the radio station. "And I told him that in our situation it would probably bode better for the organization not with him but without him. With that said, if he's with us, then we'll work around him. We'll hope he puts up the kind of numbers that we hope he can and we'll see where it goes from there."