Clayton becomes the third St. Louis wide receiver to suffer a season-ending injury in 2010. Last year's leading receiver, Donnie Avery, tore his ACL in the preseason. Undrafted rookie free agent Dominique Curry, who had become a contributor on special teams, tore his ACL in week three.
"We certainly don't feel good about losing Mark, because he's made a lot of plays for us, but we have to find a way to overcome that somehow," Spagnuolo said.
Clayton was acquired in a trade with Baltimore on Sept. 6 and quickly became one of rookie quarterback Sam Bradford's favorite targets.
The 28-year-old, a first-round pick in 2005, caught 22 passes for 300 yards and two touchdowns in the Rams first four games and was the Rams' leading receiver in each of those categories.
With Clayton out, the St. Louis receiving corps is reduced to four players on the active roster — Laurent Robinson, Danny Amendola, Brandon Gibson and rookie fourth-round pick Mardy Gilyard — with a combined 183 receptions and six touchdowns in 64 career NFL games.
Clayton had 257 receptions and 14 touchdowns in 81 career games over his six seasons, including five with the Ravens.
Spagnuolo said during his Monday press conference that practice squad wide receiver Danario Alexander, the undrafted rookie from the University of Missouri, could be a possibility to fill that roster spot.
Alexander, who had 113 receptions for 1,781 yards and 14 touchdowns during his senior season in 2009 at Missouri, is working his way back after knee surgery in the spring. There are still concerns about the strength of the area around Alexander's knee.
"We'll certainly look at Danario," Spagnuolo said. He's excited about doing it. We want to make sure he's OK physically. Then I haven't had a chance to talk to Billy (Devaney) about other names.