Shearer has described the timing of Sunderland's decision to part company with manager O'Neill as "bizarre".
Speaking on BBC1's Match of the Day, the former Newcastle striker and manager said: "It's bizarre. They have gone down the route that Reading have (sacking Brian McDermott).
"I look at him in the last few weeks and normally he comes on telly, we see him upbeat, looking forward to things.
"To me he's been down, he's not been his usual, energetic self - whether he's known that something is going on behind the scenes I don't know.
"They are on a terrible run of form but it is the timing of it I find hard.
"I think he might have missed his right-hand man John Robertson, who for the first time hadn't been with him at his side.
"For whatever reason didn't choose to go to Sunderland like he'd been with all his other clubs with him so I think he has probably missed him as well."