The teams both enter the competition at this stage and in the midst of unbeaten runs in their respective pushes for promotion to the Premier League, with the visitors sat second in England's second tier.
Match preview
Millwall enter this season's FA Cup in the third round hoping to improve on last year's display, when they were quickly dumped out by Crystal Palace in a 2-1 third-round defeat at the Den.
They will be confident of doing so, given their form since the return to Championship action from the World Cup break, as Gary Rowett's side have improved their standing in the fight for playoff places with a four-game league unbeaten run, firstly bouncing back from a defeat at Sunderland and a draw at home to Wigan with an impressive 2-0 triumph away at Watford.
Andreas Voglsammer and Zian Flemming netted goals either side of half time in that contest on Boxing Day, and the Lions would go on to play out a goalless draw with Bristol City before beginning the New Year with another three-point haul last time out.
They hosted Rotherham United on New Year's Day and quickly went ahead through Tom Bradshaw, before a Cameron Humphreys own goal and a second for Bradshaw in the final 10 minutes saw Gary Rowett's men climb up to sixth spot in England's second tier with a 3-0 victory.
Now unbeaten in four matches and with confidence from being in the playoff places for the first time since mid-August, Millwall will look to keep building momentum in their break from Championship action and advance in the FA Cup with a home win at the weekend.
They face a Sheffield United side who are also putting an impressive run of league form on hold, with the Blades sat second and in a strong position in the fight for automatic promotion.
Going into the World Cup break on the back of a win away at Cardiff City, Paul Heckingbottom's side picked up where they left off, quickly making it four wins on the bounce as they defeated Huddersfield Town, Wigan Athletic and Coventry City on their return in December, before adding to their run with a 2-1 triumph away at Blackpool soon after Christmas.
Sander Berge and James McAtee had them two goals ahead and they stood strong to see out another win after Marvin Ekpiteta pulled one back for their hosts, before the Blades looked set to lose their five-game unbeaten run away at Queens Park Rangers as they went into injury time 1-0 down following Ilias Chair's early goal.
Sheffield United did manage to rescue a point through John Egan in the 95th minute, though, extending their run without a defeat in England's second tier and ensuring their lead over third place sits at nine points, with the South Yorkshire outfit now given a brief bit of respite after a packed league schedule.
Having also crashed out of the FA Cup in the third round last season, following a 3-0 beating away at Wolverhampton Wanderers, the Blades will have their sights set on improving this time around and will hope to begin their cup campaign with what would be an impressive away win in the capital.
Millwall form (all competitions):
D
L
D
W
D
W
Sheffield United form (all competitions):
W
W
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W
W
D
Team News
Given the busy schedule of Championship fixtures over the festive period, both teams may look to make several changes on Saturday, with the likes of Tom Bradshaw, Zian Flemming and Andreas Voglsammer likely to be given rests for Millwall.
Ryan Leonard is also set to miss out as he continues to work his way back from an injury, while Callum Styles earned a first start since his own return last time out and could be given more crucial game time at the weekend to aid his full recovery.
In the expected absence of Bradshaw from the start, Benik Afobe should come in to lead the line, while Tyler Burey is also set to be given a chance to impress in attack, as are Leeds United loanees Charlie Cresswell and Jamie Shackleton in defence and midfield respectively.
Sheffield United remain without the likes of Jack O'Connell, Rhys Norrington-Davies, Max Lowe, John Fleck, Ben Osborn and Rhian Brewster through injuries, while Oli McBurnie, who has netted eight league goals so far this season, remains a doubt and Heckingbottom is likely to make further rotations where his remaining squad depth allows.
Star forward Iliman Ndiaye is bound to sit out alongside experienced striker Billy Sharp, with Daniel Jebbison set to lead the line from the outset, while Reda Khadra is unlikely to feature in attack with his loan deal from Brighton & Hove Albion expected to be cut short.
Instead, James McAtee may come in from the outset, as could young midfielder Andre Brooks, while Chris Basham, Jayden Bogle and Jack Robinson should all come into the back line from the start to allow for further resting of key players.
Millwall possible starting lineup:
Bialkowski; McNamara, Cooper, Cresswell, Wallace; Saville, Shackleton; Styles, Honeyman; Burey, Afobe
Sheffield United possible starting lineup:
Davies; Basham, Ahmedhodzic, Robinson; Baldock, Doyle, Berge, Bogle; Brooks, McAtee; Jebbison