It was announced on Monday that Kelce suffered a partial ACL tear and a full MCL tear. He left Sunday's game against the Ravens with a knee injury.
In a conference call on Tuesday, Eagles head trainer Rick Burkholder said that an examination revealed the extent of the damage. At that point, doctors decided to surgically repair Kelce's ACL.
Burkholder said he hoped Kelce could recover within nine months and be ready for training camp in 2013.
To replace Kelce, the Eagles announced the signing of center Steve Vallos.
Vallos was signed by the Eagles in March of this year, but had been released in the final roster cutdown.
Vallos has played in 40 career games (8 starts) with Seattle (2008-09) and Cleveland (2010-11). He was originally a seventh-round draft choice of the Seahawks in 2007 and spent that season on the team's practice squad.
Vallos played collegiately at Wake Forest, where he set a Demon Deacons record for most career starts with 48.
Dallas Reynolds replaced Kelce on Sunday, taking his first NFL snap after spending three seasons on the practice squad.