I've only caught a short replay but wasn't Wee Mac off side & attempting to play the ball but missed for the on-side Krishna to slot home? If so I think the linesman is fair to stop the play as Wee Mac was interferring in play.
Don't think that is the one being debated.
He was onside IMO, probably not as clear cut as people think because of the way the wide angle lens distorts the frame, but onside none the less. It was a marginal call though.
I'd like to see a replay of the offside where Kosta was one on one with the keeper.