A 37th-minute effort from Hayden Hackney proved to be the difference between the two sides at the Riverside Stadium in the first leg, and Michael Carrick's side therefore have an advantage heading into this clash.
Match preview
Chelsea had 18 shots in the first leg at the Riverside Stadium, with Cole Palmer having enough chances to score a hat-trick, but the Blues were made to pay for poor finishing, so they have work to do in order to advance to the final of this season's EFL Cup.
There is no getting away from the fact that it has been a disappointing season for Chelsea, but they managed to record a 1-0 win over Fulham in their last match, with Palmer's first-half penalty securing the points to move the capital outfit into ninth position in the Premier League table.
Mauricio Pochettino's side are only actually one point behind seventh-placed Manchester United in the top flight, while they have won four of their last five games in all competitions, including a 4-0 success over Preston North End in the third round of the FA Cup.
Chelsea will meet Aston Villa in the fourth round of the FA Cup on Friday, with their next Premier League match not coming until January 31 at home to Liverpool.
The Blues, who have won the League Cup on five occasions, have overcome AFC Wimbledon, Brighton & Hove Albion, Blackburn Rovers and Newcastle United to advance to the semi-finals of this competition.
Middlesbrough know that they will be up against it on Tuesday night, but Hackney's cool finish from an Isaiah Jones cross in the first leg has given the Championship outfit something to hold onto heading into this match.
Boro have also enjoyed previous success in the League Cup, winning the competition in 2004, beating Tottenham Hotspur and Arsenal in the quarter-finals and semi-finals respectively before overcoming Bolton Wanderers in the final, so they have previous at this stage of the tournament.
Carrick's side, who have beaten Huddersfield Town, Bolton, Bradford City, Exeter City and Port Vale in this season's EFL Cup, will enter this match off the back of a 1-1 draw with Rotherham United in the Championship on Saturday, with the result leaving them 11th in the table.
Middlesbrough finished fourth in the Championship last season, but they were beaten by Coventry City in the semi-finals of the playoffs to keep them in the second tier for at least another campaign.
Boro are only three points off the playoffs at this stage, and a top-six finish will be their aim once again, but their full focus is currently on Tuesday's game at Stamford Bridge, which is shaping up to be a fascinating affair.
Chelsea EFL Cup form:
W
W
W
W
L
Chelsea form (all competitions):
L
W
W
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L
W
Middlesbrough EFL Cup form:
W
W
W
W
W
W
Middlesbrough form (all competitions):
W
L
L
W
W
D
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Team News
Chelsea have had substantial injury problems throughout this season, and the Blues will once again be missing a number of first-team players for the second leg of their EFL Cup semi-final.
Wesley Fofana, Reece James, Marc Cucurella, Romeo Lavia, Christopher Nkunku, Robert Sanchez and Lesley Ugochukwu are all injured, while Nicolas Jackson is representing Senegal at the Africa Cup of Nations.
Ben Chilwell featured off the bench against Fulham last time out, as he made his return from a long-term hamstring issue, but Pochettino will want to be cautious when it comes to his fitness.
There might be just the one change to the side that started against Fulham - Armando Broja potentially dropping out to allow Noni Madueke to come into a wide area, with Palmer going through the middle.
As for Boro, Darragh Lenihan, Tom Smith, Emmanuel Latte Lath and Alex Bangura are unavailable for selection due to injury problems.
Meanwhile, the visitors are without three further players due to international commitments, with Seny Dieng representing Senegal at the Africa Cup of Nations, and Sam Silvera and Riley McGree in action for Australia at the Asian Cup.
Latte Lath's injury could mean that Sam Greenwood operates through the middle, with Morgan Rogers playing in a deeper area.
Jonny Howson should be fit after missing out against Rotherham with a knock, but the Championship outfit face an anxious wait when it comes to Jones, who was forced off on Saturday with a hamstring issue, making him a major doubt for this game.
Chelsea possible starting lineup:
Petrovic; Gusto, Disasi, T Silva, Colwill; Caicedo, Fernandez; Madueke, Gallagher, Sterling; Palmer
Middlesbrough possible starting lineup:
Glover; Ayling, Fry, Clarke, Engel; Barlaser, Howson; Hackney, Rogers, Azaz; Greenwood
We say: Chelsea 2-1 Middlesbrough (Chelsea to win on penalties)
The winner of this tie will take on either Liverpool or Fulham in the final of the competition, and it would be some turn-up if Fulham and Boro could get the job done to lock horns for the trophy.
We are expecting Tuesday's second leg to be a nervy affair, with Boro good enough to score, but Chelsea should be able to navigate their way into the final, potentially via a penalty shootout.