The Dundee United boss is still fuming that Christian Doidge was allowed to put Jack Ross' team 2-0 up in Saturday's Scottish Cup semi-final.
The Welshman was clearly two yards ahead of the Tangerines backline as he ran on to Martin Boyle's pass to score and the decision by referee Bobby Madden and his assistants to award the goal has sparked fresh calls for VAR to be introduced in the Scottish game.
But Mellon insists the officials should have spotted Doidge was well offside without the aid of a replay.
"I think that decision was so bad you didn't need VAR," he said. "That's a linesman's decision - Doidge is more than a body offside.
"It's a big, big game, but that didn't even need VAR. That would have wasted VAR's time. VAR is for borderline, finger-nail stuff.
"That was a poor decision in a big game that gave us an awful lot to do to try to come back.
"The boys kept going and every credit to them because right away they knew it was offside. They all put their hands up.
"To ask if we need VAR? I think that's a different conversation. But you don't need VAR for decisions like that."
Mellon admits he was left "devastated" as the final whistle killed of hopes of ending his first year at Tannadice with a trophy.
And he knows he needs to give his players tine to come to terms with the disappointment too.
"You can't flick it off and on like a switch," he said ahead of Wednesday's Premiership clash with Motherwell. "You need to give them time to recover.
"It's a bit like mourning. You just need to give it time and accept that there will be disappointment.
"You just can't come in this morning and tell them to snap out of it.
"They will recover as time goes by. Tomorrow will be better and by the time Wednesday's game comes around they will get it completely out of their system."