The moves were made day after Buffalo opened its preseason with a 17-10 loss to Minnesota.
Watkins's future in Buffalo had already been in question after the Bills declined in May to pick up his fifth-year option.
Gaines was valuable defensive back in the Rams' rotation.
If Watkins is healthy (which is a big if considering the past two seasons) they have a solid No. 1 receiver to anchor their receiving corps. He caught 28 passes for 430 yards and two scores in his injury shortened season past year. Meanwhile, Darby was the Bills' second-round pick at the 2015 NFL Draft and he has started 29 games in Buffalo the past two seasons. He missed half of the 20-16 season dealing with a broken foot. Watkins required a second operation in January after aggravating the injury last season.
While Matthews won't necessarily replace Watkins in terms of role, he'll replace him in terms of production. Statistically speaking, Matthews is a more effective receiver than Watkins, who has caught 153 passes for almost 2,500 yards throughout his career. Last season, Watkins played in just eight games and totaled 430 yards and two scores on 28 catches.
Matthews had been something of a disappointment for the Eagles after coming in a second-round pick at the 2014 NFL Draft.