The trade is expected to be made official next week, once the new league year begins on March 10. McCoy's agent, Drew Rosenhaus, said on Twitter that a deal has not been finalized but discussions have been held.
McCoy, 27 in July, finished third in the NFL in rushing with 1,319 yards. He added five touchdowns and had nine runs of 20-plus yards. As a receiver, McCoy caught 28 passes for 155 yards.
McCoy is signed through 2017 and due a $9.75 million base salary in 2015 with an $11.95 million cap hit, according to Spotrac.com. Alonso is signed through 2016 and will make a $941,418 base salary with a cap hit of $1.17 million.
Rosenhaus and McCoy "will make noise about wanting an extension and/or declining to report" to training camp, Jason La Canfora of CBS Sports reports. La Canfora also reports Alonso had asked to be traded.
Over six seasons, McCoy, Philadelphia's second-round draft pick in 2009, has rushed 1,461 times for 6,792 yards and 44 touchdowns. He also has caught 300 passes for 2,282 yards and 10 touchdowns.
Alonso, 25 in August, sat out the 2014 season after suffering a torn ACL. In 2013, his rookie season, Alonso appeared in 16 games and recorded a team-high 159 tackles with two sacks, one forced fumble, two fumble recoveries and four interceptions.
The Bills finished 9-7 in 2014 and hired former New York Jets head coach Rex Ryan earlier this offseason. The Eagles finished 10-6. Both teams missed the playoffs.
- Mike Fiammetta