NEIL Harris felt his side gave Sheffield United "a helping hand" in Millwall's 2-0 defeat at Bramall Lane on Tuesday.
The Blades went ahead in the 16th minute when centre-back Jack O'Connell was left unmarked at the back post to head John Fleck's corner past Jordan Archer.
And Chris Wilder's side doubled their lead after Millwall failed to clear from O'Connell's surge forward down the left and into the box, with Mark Duffy eventually nipping in to set up Kieron Freeman for a curled finish.
The result meant Millwall couldn't take advantage of their game in hand over Southend, who remain three points above them in sixth but who now have a plus-seven better goal difference.
Millwall were better than they were in the 1-0 defeat to Swindon on Saturday, but Harris said defending set-pieces against the league leaders was their "downfall".
"We're disappointed with the result, obviously," he admitted. "I'm pleased with a lot of aspects of our performance, without a shadow of a doubt, some of it was really good.
"I just said to the guys, when you're playing against a team that's top of the league, when you play at the next level – which Sheffield United will do next year – it becomes about both boxes.
"The first goal comes from a set-play that we don't defend well enough. The second one we have three chances to clear the ball better than we did and we didn't.
"You can't give good teams and good players those kinds of opportunities. Bolton and Sheffield United are two top teams who are getting promoted this year, for me, because if you look at their set-play record, scoring and conceding, they are far better than anyone else in the league.
"That was our downfall tonight and against Bolton."
Millwall switched to 3-5-2 in the first half after going behind to match their opponents, with Shane Ferguson and Mahlon Romeo the wing-backs and Jed Wallace playing behind Lee Gregory and Fred Onyedinma.
Harris changed to 4-3-3 when Steve Morison and Aiden O'Brien came on, before Gregory sent the ball wide after getting through one-on-one with just Simon Moore to beat.
With minutes left Shaun Williams saw his shot blocked by Chris Basham, and Harris revealed it was planned to change shape against the Blades if circumstances dictated it.
"We had the mindset to that just to help defensively to deal with diagonal balls," Harris said. "I thought man for man it was very even after that, I thought we showed our ability. When we changed to 4-3-3 I thought we were even better, more effective.
"Sheffield United have good players, a really good squad but we can't give them a helping hand like we did with the two goals."
There was a further blow for Millwall's play-off chances, after Archer limped off in the first half to be replaced with Tom King. Archer is likely to be a doubt for Saturday's clash with Scunthorpe.
Image: Millwall FC