Is the referee only sent to the monitor for an offside decision if it's a judgement as to whether a player is interfering - possibly obscuring a goalkeeper's view or in this case assessed as preventing an opponent getting to the ball? Must admit when I saw Oliver go to the screen I assumed initially it was for a foul on Cavani, which turned out to be not so.
I wasn't conscious of potential pi by Shaw before he was cautioned but I agree Fernandes and the Villa player were fortunate.