Apart from fitness, stamina etc there's three aspects to assessing Graham as a player*.
1/Number of goals scored against sitters missed, positioning when scoring chances created.
2/Link up play, back to the goal stuff, assists etc.
3/Amount of quality service he receives.
*Apart from the occasional heid gone moments I doubt his captaincy is in question.
Generalising I know but I reckon No3 is in his favour. He's not receiving enough quality balls either from wide positions or thru midfield.
No 2 isn't ever Brian's strongest suit, which exasperates things if he ain't scoring.
Re No 1, because of poor service it's perhaps unfair to assess Graham's current goal conversion rate. Just not enough data.
It's all very much a management problem. Players get dropped all the time for tactical purposes even if they don't deserve to miss out. There's no reason whatsoever that makes Graham undroppable. That said it's far from clear to me whether dropping Brian is an answer.