The Indianapolis Colts might not have the best defense in the NFL, partly because the team has suffered so many injuries in the secondary, but they do certainly have one of the best defensive lines in the league. This has been the case for several years.
When a team has Grover Stewart and DeForest Buckner, havoc will ensue. The pass rush overall might be inconsistent (again, partly because of the number of injuries), Stewart and Buckner seemingly supply a lot of quarterback pressure on a weekly basis.
Buckner, specifically, was one of the bright spots on the defense in Week 7 against the Los Angeles Chargers.