Coach Brad Childress confirmed that backup defensive end Brian Robison had arthroscopic knee surgery Tuesday. Robison left last Sunday's game against the Bears for a brief time after getting hurt, but he later returned after limping off the field.
Childress declined to declare Robison out for this week's game against winless the Detroit Lions, saying "people heal differently." Robison is an end by trade but has played inside some this season.
Star defensive tackles Kevin Williams and Pat Williams were suspended for four games Tuesday by the NFL for testing positive for violating the league's anti-doping policy.
The players don't count against the 53-man active roster during their punishment. The Vikings filled one of their spots by signing defensive tackle Jimmy Kennedy, who was let go last week by the Jacksonville Jaguars.
Running back Adrian Peterson joined Robison in the training room during Wednesday's practice. He has taken a handful of days off this season to rest. Right guard Anthony Herrera was also absent; he was with family following the death of his brother.