The Newcastle United icon has been discussing his former club's terrible start to the season.
Indeed, Shearer feels the Serb, signed for £13 million this summer - as reported by BBC Sport, has only produced his best 'once or twice' thus far.
Naturally, though, the Newcastle legend's biggest concern is the back four...
Shearer is quoted as telling reporters: "I have been impressed with him. He has bullied defenders, worked hard, held it up, brought team-mates into the game and been a threat.
"But we have only seen that once or twice at the minute. It is still early days to say that he is going to be a roaring success. He has not got a fantastic goalscoring record.
"My worry with Newcastle is always defensively. They don't seem to have shored that up as yet."
Patient fans will say Newcastle's season starts here, with Norwich City up next for the Magpies.
The problem for Steve McClaren and his charges, though, is that they had the exact same opportunity to put their struggles behind them against Watford at home - during what was a break from regularly facing Champions League chasing opposition.
They duly lost that game 2-1 at St James' Park and some of Newcastle's defending was the worst the Premier League has ever seen - perhaps barring the likes of Wolves and Derby County when they got relegated.
If McClaren and his players are bothered enough to want to beat the Canaries, the two areas Shearer has singled out - Mitrovic and the defence - will be absolutely paramount.