While the Rams, captained by Wayne Rooney, failed to muster a shot on target, the biggest flashpoint came in the early stages.
Northampton skipper Charlie Goode allowed Jack Marriott to get the wrong side of him before hauling him the the ground – with Derby's bench then left up in arms as referee Darren England opted against blowing his whistle.
With a replay now due at Pride Park, Cocu will have an extra fixture to deal with on a night where he felt he was let down by the official as the Dutchman suggested the use of VAR should not be required for such a clear foul.
"The decisive moment of the game is the red card, what should have been a red card," he said.
"I don't think we need VAR for these kinds of incidents, if you don't see it you shouldn't be here leading the game. There is no doubt about it, a great performance from the referee."
Cocu also revealed he was told by fourth official Darren Drysdale that Goode – later awarded the man of the match award – should have been dismissed.
"Yes, it is a foul," he replied when asked what Drysdale had said to him.
"At least one had a good vision, I think the linesman had good vision too so I can't give you any explanation why there is no foul given because if he doesn't pull him down Jack will score."
While bemoaning the call of referee England, Cocu did admit his much-changed side were not at their best, adding: "It was the game we expected, they were very direct and full of energy.
"We struggled now and then and maybe that was our fault, tactically we did not exactly perform what we had discussed. We adjusted it and had more control of the game and I think we did better and better but couldn't get the goal.
Rooney was largely kept quiet by the Sky Bet League Two opposition, who were more than deserving of a replay after creating the better chances – including Vadaine Oliver hitting the top of the crossbar.
Cobblers boss Keith Curle agreed with Cocu's assessment of the big call of the evening but felt Northampton were good value for the draw.
"I can only say that the referee must have been unsighted," he said.
"From where I was, I thought he would have had a decision to make and it would have been quite an easy decision for him so I can only assume he was unsighted and didn't see it and if you don't see something you can't give it.
"We needed to be on the same page and I thought we were and then we needed that little bit of luck but you earn luck and we earned that draw against a very good team and we will accept that challenge of going to Derby and plying our trade again."