Oliver Norwood believes Sheffield United have the fearless mentality to keep extending their FA Cup run.
Everton loanee Mo Besic curled United into a second-half lead and breaking the deadlock allowed the visitors to control the latter stages after a stodgy opening.
The Blades now go into the draw for the last 16 on Monday, with Norwood insisting Wilder's side will relish facing anyone in the next round.
"Going to Millwall is always difficult," said Norwood.
"We spoke about a cup run before the match and we're in the hat now and that's important.
"It was a very professional performance from the boys. The mentality, you see what the group's about; nothing fazes us.
"You know what you're getting when you come to The Den.
"We rose to that and we took their game away from them, because we matched them.
"They try and intimidate you and beat you up but we stood up to that."
Evergreen striker and captain Billy Sharp played a pivotal role in the Blades' victory, laying on Norwood's goal but also leading the line as a physical presence.
Norwood admitted the 33-year-old's performance proved key to United's battling victory.
"We got a little bit of momentum after the first one," Norwood told Sheffield United's official club website.
"Then Sharpy set it up nicely for me there, I was screaming at him to just roll it to me and then he did.
"The skip then went through and I'm sure he'll be disappointed not to have scored.
"But his all-round game, I thought he was brilliant up there, he brought us a real presence at the top of the pitch and made it difficult for their two centre-halves for us."
Millwall boss Gary Rowett admitted his side were comfortably beaten, with the Lions paying the price for Aiden O'Brien's costly first-half miss.
Rowett then quickly scotched rumours he had placed a sign reading 'promotion' over the dressing room door, with the club eyeing a push towards the top end of the Championship table.
Asked about the rumours, Rowett said: "That's the stupidest rumour I've ever heard.
"If there was one up there I'd rip it straight down.
"That's obviously our ambition but for me it's about trying to build.
"We don't want to be a good Championship side one season then the next season we're struggling.
"I wouldn't even mention the word promotion, let alone put a sign above any door."