Harbaugh acknowledged receiving Willis' MRI results, but when asked by reporters to reveal them, the coach said, "No, we wouldn't share those."
Pressed on whether the four-time Pro Bowl linebacker's injury was a strain or worse, Harbaugh said, "Wouldn't share any information about it."
Finally, when asked if he expected Willis to play this weekend against the Arizona Cardinals, Harbaugh answered, "Don't know, don't know yet. Can't make that determination today. He's not going to practice today. But, that doesn't mean he won't play Sunday."
Willis was at Wednesday morning's walk-through session, but he wasn't on the field for the start of the afternoon practice, according to The Mercury News.
Cornerback Carlos Rogers didn't practice because of a knee injury, according to the team's injury report. His status for Sunday's game wasn't known.