The home side narrowly progressed to the second stage after beating Portsmouth 1-0 on August 13, while the visitors comfortably advanced after beating Newport County 4-1 on the same date.
Match preview
Romain Esse scored the only goal of the game in the 13th minute of the hosts' victory against Portsmouth when he finished smartly after George Honeyman won back possession close to the opposition box.
Although Millwall managed to hold on and secure the victory, Portsmouth had more possession, more shots and created more big chances.
The Lions have made a poor start to their 2024-25 league season and have accumulated just one point from three games, seeing them sit third last in 22nd place.
Millwall have had success from an offensive perspective considering they have scored five goals and rank first in the Championship for big chances created, but they have also conceded seven times, the joint most in the division.
Manager Neil Harris noted that he was happy his side were able to earn their first point and clean sheet of the season against Hull last time out on August 24 but thought his team had the better chances and deserved to win.
Opponents Leyton Orient were 3-0 up against Newport County in the first round of the EFL Cup before they conceded in the 61st minute, but a 74th-minute strike from Charlie Kelman restored their three-goal advantage.
Despite only having 49% possession, the Os had 10 shots and nine efforts from inside the penalty area, stats which were double and triple that of Newport County respectively.
However, their strong showing in the EFL Cup's first round is not reflective of their league campaign considering they have lost all three of their matches in League One and are currently 23rd.
Leyton Orient worryingly rank second last for chances created, 16th for touches in the opposition box and 21st for xG created.
Boss Richie Wellens's side come into the clash having failed to progress from seven of their last 10 ties in the EFL Cup, as well as four of their last five away matches in the competition.
Millwall EFL Cup form:
W
Millwall form (all competitions):
Leyton Orient EFL Cup form:
W
Leyton Orient form (all competitions):
Team News
The hosts will be unable to select midfielders Billy Mitchell and Zian Flemming, who are out with hip and calf injuries.
Millwall could name a similar team to the one that beat Portsmouth, so expect goalkeeper Liam Roberts to be shielded by a defence of Danny McNamara, Wesley Harding, Jake Cooper and Joseph Bryan.
As for the visitors, they may start Daniel Agyei, Diallang Jaiyesimi and Brandon Cooper given all three scored against Newport County.
Elsewhere, Leyton Orient could name Ethan Galbraith, Lewis Warrington and Zech Obiero in midfield.
Millwall possible starting lineup:
Roberts; McNamara, Cooper, Harding, Bryan; De Norre, Massey; Emakhu, Honeyman, Esse; Langstaff
Leyton Orient possible starting lineup:
Hemming; Clare, Cooper, James, Sweeney; Warrington, Galbraith, Obiero; Jaiyesimi, Agyei Perkins
We say: Millwall 4-1 Leyton Orient
While Millwall have been defensively vulnerable this season, they have at least been strong from an offensive perspective, and they could threaten Leyton Orient's goal on Tuesday.