The 20-year-old ruptured his anterior cruciate ligament, shortly after scoring his first professional goal in the Railwaymen's 2-2 draw at Rochdale on Tuesday night.
Seager has returned to his parent club after making four appearances for the League One club.
A statement on Crewe's official website read: “Everyone at Crewe Alexandra would like to thank Ryan for his contributions to the club and wish him well with his recovery."
The striker has expressed his gratefulness to the club on his Twitter account:
It's been a fantastic experience and I'll never forget it, good luck to all the players, staff and fans! Thanks for being so supportive
— Ryan Seager (@Ryan_Seager) February 18, 2016