Lee Gregory and Aiden O'Brien had given the Lions a two goal lead against a Bees side that was strangely flat.
But an Alex Pritchard penalty and Moses Odubajo struck in the last five minutes to level the scores at the death.
He said: “It's two points dropped but it could be a very valuable point come May 2.
“We were below our best today and we've got no divine right to get three points here. We have to recognise today wasn't good enough for us.”
The Bees were predictable in the first half, breaking down the left flank and then trying to curl a right footed effort in at the far post as Gregory capitalised on slack defending to give Millwall the lead.
Although they improved in the second they did not look likely to score and were 2-0 down after O'Brien's effort.
They were handed a lifeline when Alan Judge was felled in the box with Pritchard slotting home and Odubajo levelled after the Spurs loanee's cross.
Warburton denied suggestions his side felt they had to merely turn up to claim the win.
He said: “We knew Millwall would be scrapping hard and we started in a scrappy manner. We never dominated the football like we do and paid the price.
“We put too many aimless crosses into the box which is very unlike us and the players know that's not the level but we've got to recognise what we do well.
“We took more care in the second half but we were sloppy at the back. It was below standards today all round.
“We had great character to respond as we did get an important point.”
It's the third time this week Brentford have conceded twice but Warburton is not concerned about the porous nature of the back four.
He added: “Not when you score three. The other night we scored three and we nearly got there at the end.
“Tommy Smith should have won a penalty. How Tommy gets booked I don't know but we had a few chances in the second half.
“It wasn't good enough overall from players and staff.”