Coach Chuck Pagano confirmed the diagnosis during his regular Monday news conference.
Losing Castonzo would be another big blow to an offense that is already without Andrew Luck, who is expected to miss most of the rest of the regular season with a lacerated kidney and partially torn abdominal muscle.
The offensive line has struggled to protect Luck and his replacement, Matt Hasselbeck, even though the 40-year-old Hasselbeck improved to 3-0 with Sunday's win at Atlanta. Castonzo has been the one consistent performer on the line. He had started 66 consecutive regular-season games and signed a four-year, $43.6 million contract extension in September.
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