Heading into this knockout match Chelsea are unbeaten in their last 10 games in all competitions, while the Posh have lost two of their previous three games, falling 1-0 to Bristol Rovers on Saturday.
Match preview
Some fine reactions in the group stage enabled Mark Robinson's men to claim victory in their final two encounters, as they wound up topping Group G.
They failed to score a single first-half goal in the group portion of this competition, conceding within the opening 10 minutes of their first two matches.
Since a 2-1 victory over Oxford United in early October, the Blues have been much sharper in front of goal, scoring three times in four of their last five outings.
Chelsea have not scored a goal in the opening half since drawing the Leicester City U-21s at the beginning of October, though they have not conceded a first-half strike in their last four fixtures.
They have made it to the second round of this tournament in two successive campaigns, defeating Forest Green Rovers on penalties last season before losing to the Arsenal youth squad in the next round.
Some late heroics in their final group stage encounter proved decisive as Peterborough equalised in the 83rd minute against Wycombe Wanderers, edging them for second place in the group on goal difference.
Grant McCann is in his third spell as manager of this club, and lately, consistency from one match to another has been a problem, with Peterborough failing to win consecutive games since late October.
Despite those inconsistencies in recent times, this team have found a way to contend for the League One title often, finishing in the top seven or higher over the past four domestic campaigns.
They sit fourth in the domestic table, thanks to their quality in the attacking third, scoring the fourth-most goals in League One this season.
Tuesday will be their first appearance in the second round of this tournament since 2020-21, with Peterborough unbeaten after 90 minutes at this stage since 2008-09.
Chelsea Under-21s EFL Trophy form:
L
W
W
Chelsea Under-21s form (all competitions):
W
D
W
W
D
W
Peterborough United EFL Trophy form:
L
W
L
Peterborough United form (all competitions):
W
W
D
L
W
L
Team News
Malik Mothersille, Charlie Webster and Jude Soonsup-Bell all found the back of the net for Chelsea in their 3-0 win over the Tottenham U-21 team, while Eddie Beach picked up a shutout.
Mothersille had a pair of goals in their last group stage victory over Leyton Orient (3-2), with the opener coming courtesy of Omari Hutchinson.
Zak Sturge, Derrick Abu, Charlie Webster, Cesare Casadei and Soonsup-Bell were all newcomers to the starting 11 versus Spurs.
Jack Marriott had a brace in Peterborough's FA Cup replay victory over Salford City (3-0), as Lucas Bergstrom, who is on loan from Chelsea, collected a clean sheet in both of those outings.
A late strike from captain Jonson Clarke-Harris enabled them to force penalties in their last group fixture versus Wycombe, but Kelland Watts, Joe Ward and Ronnie Edwards all missed from 12 yards away.
We saw the same starting 11 for them this weekend that began their FA Cup replay encounter, though there were brand new substitutes that featured on Saturday, including Clarke-Harris, Ricky-Jade Jones, Joel Randall, Ben Thompson and Josh Knight.
Chelsea Under-21s possible starting lineup:
Beach; Sturge, Gilchrist, Abu; Williams, Webster, Elliott, Rankie; Casadei; Mothersille, Soonsup-Bell
Peterborough United possible starting lineup:
Bergstrom; Butler, Edwards, Kent, Thompson; Kyprianou, Fuchs; Randall, Burrows, Poku; Mariott
We say: Chelsea Under-21s 1-1 Peterborough United (Chelsea advances on penalties)
Chelsea are in fine form and do not appear to be bothered if they go behind, plus their quality and experience in these elimination games should propel them to a narrow victory this week.