Eagles wide receiver Jeremy has torn the ACL in his right knee and could miss the season.
Maclin went down at the end of a non-contact drill Saturday in the team's second full practice under new coach Chip Kelly and appeared in pain while on the ground for several minutes.
He was helped onto the cart and needed assistance getting off it and going into the team's practice facility.
The first practice with pads is Sunday.
Maclin is headed into his fifth season with the Eagles.
He led the team with 69 catches for 857 yards and seven touchdowns last season.
The Eagles signed wide receiver David Ball to the 90-man roster.
The 29-year-old Ball played for Kelly at New Hampshire, and broke Jerry Rice's Division I-AA record for touchdown receptions.