The hosts beat Bristol Rovers 2-0 to progress into the second round, while this match will be the first EFL Cup fixture for the visitors this term.
Match preview
Cardiff City have had a disappointing time in the seasons following their relegation from the Premier League in 2018-19, rapidly slipping down the table in the subsequent years.
They placed fifth in 2019-20, eighth in 2020-21, 18th in 2021-22 and reached a low of 21st in 2022-23, finishing narrowly above the relegation zone with just a five-point lead.
Cardiff were improved in the most recent season, climbing up the table to finish 12th with 62 points, leaving them 11 points adrift of the playoff places and 12 above the drop zone.
Erol Bulut, who took charge of the team ahead of the 2023-24 campaign, would have been aiming to continue that progress this season, and a promising pre-season that saw Cardiff win three and draw two of five friendly matches would have provided optimism ahead of their opening fixture.
However, the Bluebirds have lost two and drawn one of their first three Championship games, with their only victory coming against Bristol Rovers in the first round of this competition.
A late draw against rivals Swansea in their most recent outing could boost spirits in the Cardiff squad, and Bulut will be aiming to build on that with a positive performance against Southampton.
The visitors have also had a challenging opening to their return to the Premier League, losing 1-0 to both Newcastle United and Nottingham Forest, leaving them as one of five teams yet to acquire a point in the top division.
Russel Martin, who led Southampton to promotion from the Championship in their first attempt last year, would have been encouraged following their performance against Newcastle, where the Saints had 78% possession and took 19 shots to the Magpies' three.
However, they produced an extremely underwhelming showing against Nottingham Forest as despite holding 65% possession, they took just five shots to their opponents' 23, leaving much to be desired.
Martin will be aiming not only for an improved performance against Cardiff on Wednesday but also for a first victory of the season, with a favourable run of fixtures ahead, including matches against Brentford, Ipswich Town, and Bournemouth in their next four games.
Cardiff City EFL Cup form:
W
Cardiff City form (all competitions):
Southampton form (all competitions):
Team News
Cardiff are expected to be unable to call upon Joe Ralls, Isaak Davies, Jamilu Collins, Kion Etete, Ryotaro Tsunoda and Will Fish for this match due to ongoing injury issues.
Callum Robinson scored Cardiff's first league goal of the campaign against Swansea last time out, and the striker is likely to come into the starting team in place of Wilfried Kanga here.
As for Southampton, Adam Lallana, Kamaldeen Sulemana and Ross Stewart are all doubts for the match, but the trio could return to the side in time for their game against Brentford.
Gavin Bazunu is ruled out of action until late October due to an achilles injury, while Juan Larios is also out until early September with physical discomfort.
Cardiff City possible starting lineup:
Alnwick; Ng, Chambers, Daland, O'Dowda; Siopis, Robertson; El Ghazi, Ramsey, Willock; Robinson
Southampton possible starting lineup:
McCarthy; Harwood-Bellis, Bednarek, Stephens; Sugawara, Dibling, Downes, Aribo, Walker-Peters; Brereton Diaz, Archer