Goals from Jack O'Connell and Kieron Freeman condemned Millwall to defeat at Sky Bet League One leaders Sheffield United on Tuesday night.
Following last Saturday's 1-0 defeat at Swindon Town, Neil Harris made three changes to his starting line-up, with Ben Thompson, Shane Ferguson and Fred Onyedinma replacing Calum Butcher, Aiden O'Brien and Steve Morison, who all dropped to the bench.
The Lions carved out the first opportunity of the game in the opening stages through good work by Ferguson and Lee Gregory, but Simon Moore claimed the former's cross with Onyedinma lurking at the back post.
The visitors started the brighter and made most of the attacking play, but could only end it on 10 minutes with a shot from Mahlon Romeo which was dragged wide.
It was to be Sheffield United, however, that would take the lead five minutes later.
After a couple of heroic blocks from Lions defenders, the resulting corner-kick was swung in towards the head of Jack O'Connell, who headed home to open the scoring.
Moments later, Archer was on hand to pull off a stunning save from Paul Coutts' long-range effort as The Blades looked to quickly double their lead.
Sadly, that was to be the goalkeeper's last real involvement in the game, as he had to be replaced by Tom King on 26 minutes due to injury.
Substitute King had little to do for the remainder of the first-half, with a James Hanson header - which rolled wide - the only United shot of note.
At the other end, meanwhile, The Lions huffed and puffed, but were not able to find a way past a resolute Blades' back-line.
Ten minutes into the second-half, The Blades' lead was doubled through Kieron Freeman.
Indecision in the Millwall defence saw the ball cannon to Freeman, who curled past the dive of King to put United two to the good.
Harris introduced Morison and O'Brien into the play as The Lions looked to find a way back. Neat play between the latter and Wallace nearly set the loanee on goal with 20 minutes remaining, but a United tackle just nipped the ball from his toe as he shaped to shoot.
Two minutes later, Wallace's through ball allowed Gregory to set his sights on goal, but he curled his shot wide of the post.
Nine minutes from time, Shaun Williams almost halved the deficit, but his goal-bound attempt was blocked by a Blades leg.
Millwall were perhaps on top of their top-of-the-table opponents for large parts of the second-half, but were unable to find a way back into the game, succumbing to defeat.
Sheffield United: Moore, Fleck, O'Connell, Basham, Sharp (O'Shea, 83'), Wright, Coutts, Hanson, Freeman, Duffy (Carruthers, 67'), Lafferty. Subs not used: Long, Done, Wilson, Clarke, Chapman.
Bookings: Wright, Basham
Scorers: O'Connell (16'), Freeman (55')
Millwall: Archer (King, 26'), Craig (O'Brien, 66'), Williams, Thompson, Gregory, Onyedinma (Morison, 51'), Romeo, Wallace, Webster, Ferguson, Cooper. Subs not used: Cummings, Hutchinson, Worrall, Butcher.
Bookings: Thompson, O'Brien
Referee: J. Simpson
Attendance: 20,832 (345)