A source told ESPN's Adam Schefter that Bosa visited with foot specialist Dr. Robert Anderson on Wednesday and that he has a bone bruise in his injured left foot.
Bosa is not expected to play Sunday at the Buffalo Bills and could be out even longer, the source said.
The Chargers ruled out Bosa for last week's season opener against the Kansas City Chiefs due to the injury. Los Angeles coach Anthony Lynn said last week that Bosa's foot was not broken and that he would be considered week-to-week.
According to the Chargers, Bosa's initial left foot injury suffered a month ago had healed, but he sustained a new injury on the same foot during practice last week. Bosa was on the field going through individual drills and felt something wasn't right, according to Lynn.
Bosa suffered the initial injury, described as a sore foot by Lynn, on Aug. 7 during 11-on-11 team drills in practice. Afterward, Bosa hobbled off the field on his own and spent the rest of practice receiving treatment from the team's training staff.
Bosa did not participate in any preseason games. However, the Chargers considered his injury minor and the expectation was he would be ready for the beginning on the regular season.
That all changed in the lead-up to the team's opener at home against the Chiefs, when Lynn said Bosa might not play after being a limited participant in practice on Sept. 5.
Bills coach Sean McDermott said he doesn't expect Bosa's absence to significantly impact Sunday's contest.
The Chargers easily handled the Bills 54-24 last year at StubHub Center, intercepting Nathan Peterman five times in the first half in his first start.
Peterman has been replaced in the lineup this week by rookie first-round draft pick Josh Allen, who will make his first NFL start against the Bolts.
"I don't think it changes much," McDermott said about Bosa not expected to participate on Sunday. "They've got talented players over there, still. We are very aware of their personnel. They've got good personnel out there, and I think they are a talented football team."
The No. 3 overall selection of the 2016 draft, Bosa led the Chargers in sacks last season with 12.5, and he is one-half of the Chargers' talented pass-rush tandem that includes Melvin Ingram, who finished with 10.5 sacks last year.