The Lions will face a familiar foe when they host Sheffield United in Saturday's Sky Bet Championship fixture at The Den.
The newly-promoted Blades, who sit third after an impressive start to life in a new division, will travel to The Den seeking a return to winning ways after a narrow defeat at Fulham and 1-1 draw with Birmingham.
"They have carried the momentum from last year and have done really well, Neil Harris told millwallfc.co.uk ahead of the match.
"We both came up from League One last season. The difference between us and them early in the season has been that they have had two scorers that are in red-hot form this year. We certainly have to be at our best to keep them quiet.
"I would class Sheffield United as a big team at this level. It has taken them a few years to get back up and they won the League last year - and rightly so.
"They have consistency in their group and are performing well. Are we they underdog? Yes, the League position says that we are, but we are playing at The Den and I would never say we are an underdog to anyone at home.
"We certainly look forward to putting them under pressure and putting in a strong performance."
Millwall winger Jed Wallace is expected to make a return to match action after serving a three match ban for being sent off in the defeat to Burton Albion last month.
"He certainly has the most creativity in our team and we certainly missed that," Harris added.
"I think our shape without the ball has been outstanding recently, they certainly don't lack any desire, passion or purpose but in the final third we have perhaps lacked that creativity.
"Jed certainly provides that spark when he plays and the fans certainly enjoying watching him."
Lions right-back Conor McLaughlin will miss the fixture after picking up a fifth yellow card in the defeat to Fulham last weekend, possibly opening the door for Mahlon Romeo to return to the starting side.
"Mahlon was outstanding last year and has patiently waiting for his chance to come back in," Harris added.
"The loss of Conor also gives me the option to change the shape of the team. There's a lot to ponder before the weekend.