I think the rule is the keeper needs to have the ball under control. Although Adrian had two hands on the ball I don’t think he ever had it under control. Seemed a fair goal to me.
(Unbelievable how opaquely the laws are written.)
This is from the Twitter VAR nerd Dale Johnson
https://twitter.com/DaleJohnsonESPN/status/1554069359307505667?s=20&t=UHHXuTg47rhvP6wiTZW-ugBy his interpretation, because it's a rebound from a keeper's spill (unlike the Schmeichel incident last December) having it under control is not the issue, the issue is whether there's a foul on Adrián. (At least I think that's what he means.)
In my opinion, there clearly is a foul, but I also think that referees are hard on players who are a bit crap, e.g. Adrián, who is well-known to be a bit of a disaster. You get punished for spilling a shot inside your own six-yard box. It's like how Xhaka gets sent off for every rough tackle where other players with less bad reputations get away with it: it fits the pre-conceived narrative. To add to that narrative factor, Adrián probably would've gotten away with it if he'd have rolled around and pretended he was really hurt, or pretended it was a head injury.
From my point of view, it's a clear foul on Adrián, but Adrián has to do better to avoid getting in that situation and not leaving it in the hands of the refs who are shite. Mind you, he's our third choice keeper, so I don't expect much more and thought he did OK in the rest of the game. Scott Carson is City's third choice keeper and their second choice keeper (Steffen) wasn't much cop either in the FA Cup semi-final.
Also the linesman shouldn't be flagging in that situation having spent the last two seasons not flagging anything even when it was clearly offside. When they flag now, players will expect that it's not even close to being onside. (I think this is a cock-up rather than a change in procedure though.)