Pompey head into the first-round fixture off the back of an entertaining draw in their league opener against Leeds United, while Millwall began their Championship campaign with a narrow defeat to Watford.
Match preview
After winning the League One title last season, Pompey made an exciting start to their first Championship campaign since 2011-12.
John Mousinho's side fell behind in the 10th minute of Saturday's away clash with Leeds United, before Elias Sorensen and Callum Lang grabbed a goal apiece to give Pompey a slender lead at the interval.
Although they conceded in the first minute of the second half, Portsmouth looked set to claim a dramatic victory when Lang netted a 92nd-minute penalty, only to see his effort cancelled out by Brenden Aaronson's late equaliser.
Despite missing out on all three points, Pompey would have been satisfied with a draw in their tricky opener, giving them a platform to build on in their next league game against recently relegated Luton Town.
However, before they can prepare for a home meeting with the Hatters, Portsmouth will attempt to reach the second round of the EFL Cup for the third season in a row.
Incidentally, Pompey's most recent first-round exit took place in their previous meeting with Millwall in August 2021 when they fell to a narrow 2-1 defeat at The Den.
After finishing the 2023-24 campaign with five consecutive victories, Millwall would have been hoping to pick up from where they left off when they played host to Watford on Saturday.
Unfortunately for the Lions, they fell to a dramatic 3-2 loss at The Den despite mounting an impressive fightback from two goals down.
Duncan scored in the 74th and 88th minute to pull Millwall back onto level terms, only to see Watford substitute Mileta Rajovic net a 90th-minute winner.
Neil Harris will hope his side can produce a positive response in the League Cup as Millwall bid to avoid a third consecutive first-round exit.
The Lions suffered a 1-0 defeat to Cambridge United in the first round in 2022-23, before they fell to a heavy 4-0 loss at this stage of last season's competition.
With Millwall looking to end their three-year wait for a second-round appearance, they may be able to take confidence from the fact they have won each of their previous three games against Portsmouth, including a 1-0 win on their most recent trip to Fratton Park in April 2012.
Portsmouth form (all competitions):
D
Millwall form (all competitions):
L
Team News
The hosts are still unable to call upon the services of Regan Poole, Jacob Farrell, Josh Murphy and Colby Bishop.
Australia international Kusini Yengi missed the league opener due to a slight groin strain, and while he is expected to be absent for Tuesday's game, Mousinho is hopeful he will prove his fitness in time for Saturday's Championship clash with Luton Town.
Summer signing Jordan Archer could be one of several players brought into the side for the cup fixture, with the ex-Millwall goalkeeper set to make his Pompey debut against his former club.
As for the visitors, they are expected to be without the injured quartet of Murray Wallace, Billy Mitchell, Adam Mayor and Zian Flemming.
Defender Japhet Tanganga is set to serve the second game of a three-match suspension after being sent off in the final game of the 2023-24 campaign.
Harris is likely to rotate his side for the EFL Cup clash, with Liam Roberts, Wes Harding, Danny McNamara, Aidomo Emakhu and Macaulay Langstaff all in contention to start the first-round fixture.
Portsmouth possible starting lineup:
Archer; Swanson, Shaughnessy, Towler, Ogilvie; Lowery, Moxon; Devlin, Saydee, Richie; Sorensen
Millwall possible starting lineup:
Roberts; McNamara, Harding, Cooper, Bryan; Saville, De Noore; Esse, Watmore, Emakhu; Langstaff