Doctors attempted to correct a nerve issue in Nelson's knee that has bothered him since his days in college, a league source told ESPN. The issue has flared up periodically and unexpectedly, including at the beginning of this summer's camp.
Nelson felt like he could have played with the pain all season, taking medication and missing practice, but he opted to undergo the surgery that he has been told will correct the problem. Packers wide receiver James Jones also underwent a similar procedure that helped him deal with the condition.
Nelson dropped out of practice late last week and did not participate in the team's scrimmage on Saturday night.
McCarthy said he is hopeful that Nelson will return in time for the regular-season opener at San Francisco on Sept. 8.
Another receiver, Randall Cobb, dropped out of Tuesday's practice because of a biceps injury.
McCarthy also said Bulaga is still waiting on a second medical opinion on his injured knee and is still hopeful that he can play this season.
ESPN NFL Insider Adam Schefter contributed to this report.