Varney received his marching orders at the start of the second half after catching Adam Smith, with the Lions finally making their numerical advantage count four minutes from time as Chris Wood nodded the winner.
Warnock, whose side have now gone seven league games without a win, said: "The amount of effort that's gone into the game and we're talking about a refereeing decision - it's scandalous.
"I thought it was harsh really. He's put his arms up, I know he has, but he's not actually gone in with his elbow like Mark Halsey says he has.
"He elbows him below the chin - the kid goes down as if he's been poleaxed in his head!
"I'm disappointed at an experienced referee like that. it's getting soft now, the game, I think. That's not a proper sending off - I don't know what the game's coming to."
Warnock, who also claimed his side were denied a "clear penalty", added: I can't fault the lads, they were great today. It was difficult with 11, but with 10...
"I thought we still pushed as best we could, left three up front and had a go, so I've no complaints with the effort today. I enjoyed it, I thought we played well."