Neil Harris says he will ring the changes for the visit of Rochdale on Saturday but has challenged his Millwall side not to pass up the chance of another Cup run.
The Lions will host the League One relegation battlers this weekend after easing past Barnsley in the Third Round.
"There will be changes to the team but it's a game that's there to be won," Harris told millwallfc.co.uk
"We want to be in the next round of the Cup but I would be lying if I said the big focus isn't on the League - playing Derby on Tuesday and then Reading next weekend.
"I have to think about the balance with team selection in preparation for that."
Loanee Jason Shackell joined Millwall earlier this week from Derby County and the defensive recruit may be unveiled against Dale.
"Jason could well come in, yes," Harris added.
"He hasn't played a lot of first-team football recently but he is fit and is available.
"We have been chasing an experienced centre-half over the course of this transfer window.
"You can never have too many experienced players in the team or in the squad to lead. I felt that in the back-line we maybe needed another voice and some guidance."