It is then up to deciding if Greer actually played the ball or deflected it by it hitting him. That is why the ref is there to make that decision and Keith Stroud is or was a Premier ref so you would expect him to get it right. I must admit to not having seen much wrong with the refs...
surely under the current offside law Greer playing it last does not put the player back onside, it did prior to the rule change in 1979 but not now. If the player was offside when the Watford player played it he was still offside after Greers touch.