The Raiders linebacker sustained a concussion at some stage during the second half, but remained on the field until the bitter end of Oakland's 35-13 loss. Team doctors diagnosed McClain in the aftermath of defeat.
"It was something we thought might have been the case after the game," coach Dennis Allen told the Bay Area News Group. "But it didn't seem like it was a major one. ... It's kind of a precautionary deal that we're going through the whole process to make sure he's fine."
McClain isn't sure when it happened. He said it's customary to absorb big hits and battle through, but "there was a time where I thought, 'I don't feel like myself.' "
His availability for Sunday's home game against the Pittsburgh Steelers is in question. McClain has yet to pass the NFL-mandated testing, meaning Travis Goethel is one step closer to starting a game Oakland must win to remain relevant in the AFC West.
UPDATE: McClain was back on the practice field Thursday, meaning he has cleared the NFL's concussion testing, the San Francisco Chronicle reports.
Follow Marc Sessler on Twitter @MarcSesslerNFL.