Perhaps he was irked by the presenter's pointed reminder that he (Shearer) had been in charge of one of the wealthiest clubs in the land when it got relegated.
I agree with all the observations about the "he must score" comments from analysts, but it is just a form of linguistic laziness. To me, it just means a chance that the players are going to shake their head over in the dressing room at half time or after the match, and think "if only".
Empirically, the pros on the show must have been in enough matches where they have missed an ostensibly simple chance, either because they were running too fast to control the ball, they were stretching, a defender did just enough to put them off or because their Chalfonts were causing them gyp or something.
What we need is a "lazy comment" filter, that works like the swear filter on Glory-Glory, changing: "he must score" to "that was a good chance"; "he'll be disappointed with that" to "that was a good chance, but I used to play in the same team as him"; "he's got to do better than that" to "I never made a single mistake in my entire career".