Noel Whelan thinks Millwall's 1-0 win over Leeds United at the Den is a warning to the Whites that teams will realise they are an attacking outfit and are take measures to stop Thomas Christiansen's side.
Leeds headed to the capital sitting top of the Championship table and not having conceded a league goal since their opening matchday win away at Bolton Wanderers.
But Leeds came undone at Millwall as Neil Harris' men were the dominant force and won the game through a 73rd minute goal from Aiden O'Brien.
Millwall also saw a Steve Morison effort ruled out for offside and hit the post through Jed Wallace, as they comfortably ended Leeds' unbeaten run this season.
Whelan feels Millwall adopted a physical approach against Leeds which the Whites could not deal with and also stressed sides will start to realise that, unlike last term, this season Christiansen's men are an attacking unit.
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"I would say it's a good day to learn, because it's quite early in the season, that this is what's going to come", Whelan said on BBC Radio Leeds.
"People will in the end realise that Leeds are an attacking side.
"They will try and be more physical with them, but they've got to be able to stand up and deal with those situations or use them to your benefit.
"Make the pitch a bit bigger, work things out on the field, how to break teams down.
"Today they just weren't at it.
"They just could not compete with how physical Millwall were and how they played the long, direct ball, squeezed and won the second balls."
Leeds will look to get back to winning ways in midweek, when they face a tough trip to take on Burnley at Turf Moor in the EFL Cup.
