Jackson suffered his injury in the first quarter. He leads the Eagles in receptions (45) and receiving yards (700). He also has two touchdown catches.
Reid said the injury could take six weeks to heal.
Jackson will be missed most in the Eagles' deep passing game. He has seven receptions of 30 or more yards this season. And according to ESPN Stats & Information's Next Level data, he's tied with the Lions' Calvin Johnson for the most 30-yard receptions in the league with 48 since his NFL debut in 2008.
Jackson's injury means the Eagles, who have lost seven straight, are without their starting quarterback, top running back and top receiver. Quarterback Michael Vick and running back LeSean McCoy both missed Monday night's loss because of concussions.
McCoy remains in Phase 1 of his concussion recovery; Vick is in the fourth of five stages, Reid said.
Vick, who has missed the past two games, will undergo another concussion test Wednesday.
The Eagles (3-8) face the Cowboys (5-6) Sunday night in Dallas.
If the Eagles fall to the Cowboys, they'll be saddled with their first eight-game losing streak since 1968, according to ESPN Stats & Information.
A loss also would tie Reid for a dubious distinction. If the Eagles' skid extends to eight, he would join Giants coach Tom Coughlin as the only active coaches to have both an eight-game winning streak and losing streak on their resumes, according to the Elias Sports Bureau. Reid's Eagles won nine straight games in 2003.
Information from The Associated Press was used in this report.