The teams announced the trade Friday evening, one day after a pair of offensive linemen suffered injuries in the Bills' preseason opener against the Indianapolis Colts.
Bates signed with the Eagles as an undrafted free agent out of Penn State this offseason after starting 12 games for the Nittany Lions in 2018. His ability to play both tackle positions was appealing to the Bills after they placed "swing" tackle LaAdrian Waddle on injured reserve earlier this week with a torn quadriceps muscle.
Harold signed a one-year deal with Buffalo in 2019 after spending his first three seasons with the San Francisco 49ers and 2018 with the Detroit Lions. He was listed as Buffalo's fourth-string defensive end on its first unofficial depth chart of the preseason. He practiced in full Friday and was informed of the trade after the conclusion of practice, according to a league source.
The addition of Harold adds needed depth at the defensive end position for the Eagles after they lost Joe Ostman to a torn ACL last week.
Waddle is the only injured Bills offensive lineman to land on IR, but center Mitch Morse has been in the concussion protocol since suffering a concussion in the team's first padded practice July 27. The Bills made Morse the league's highest-paid center with a four-year, $44.5 million deal this offseason.
Tackle Conor McDermott was cleared Thursday after being evaluated for a concussion -- but was placed into the concussion protocol Friday after showing concussion-like symptoms earlier in the day. Also missing practice were interior offensive lineman Russell Bodine, with an oblique strain, and tackle Ty Nsekhe, who was given a rest day.
Buffalo also released offensive lineman Jeremiah Sirles with an injury settlement during training camp and signed Nico Siragusa to replace offensive lineman Isaac Asiata, who announced his retirement July 30. The Bills signed offensive lineman Jarron Jones this week to correspond with Waddle's move to IR.