Idris El Mizouni’s first senior goal saw Town into the lead but Ricky-Jade Jones levelled for Peterborough to leave the scoreline 1-1 at half-time in the Leasing.com Trophy second-round tie at London Road.
Keeper Adam Przybek and central defender Alex Henderson were handed their debuts and Barry Cotter returned to first-team action for the first time since April 2018 as manager Paul Lambert named his usual much-changed side. No one who was in the XI at Coventry on Sunday is included in the 18.
Wales U21 international keeper Przybek, 19, joined the Blues in the summer after leaving West Brom following a trial last season.
Second-year scholar Henderson, 18, who is from Boxford near Sudbury and who joined Bury Town on loan at the weekend but is still able to play for the Blues, lined-up alongside stand-in skipper Cole Skuse at the centre of the defence.
Cotter - whose last first-team match was as a late sub in the 4-0 win at Reading in the penultimate match of 2017/18 - was at right-back and Myles Kenlock on the left.
The Blues again played a three-man midfield with Brett McGavin behind Emyr Huws and El Mizouni.
Up front, James Norwood played down the middle with Jordan Roberts back after his toe injury on the left and Armando Dobra on the right.
There were more youngsters on the bench, academy keeper Jake Alley, left-back Tommy Smith, 16-year-old midfielder Liam Gibbs, who made his debut as a late sub in the last round, midfielder Tommy Hughes and strikers Tyreece Simpson and Ben Folami, who all also appeared earlier in the competition. On-loan Spurs winger Anthony Georgiou was the only senior man among the subs.
Posh boss Darren Ferguson also rung the changes, making nine from the team which beat Dover Athletic 3-0 in the FA Cup on Sunday. Only Dan Butler and Joe Ward kept their places with strike pair Mo Eisa and Ivan Toney on the bench.
There was an early scare for debutant Henderson when Siriki Dembele went to ground under his attention in the penalty area in the opening minute but referee Graham Salisbury felt there was no foul.
On three, Jones headed over for the home side, then Serhat Tasdemir shot too high from distance.
Town’s first attack of the game came down the right with Cotter and El Mizouni exchanging passes before the Irishman crossed to Norwood on the penalty spot. The striker hit a shot on the turn but scuffed through to Aaron Chapman in the Peterborough goal.
In the 10th minute El Mizouni found Dobra on the right but the Albanian U19 international’s cross was too close to Chapman.
A minute later, Tasdemir hit a low curling shot from the edge of the Town box which Przybek claimed confidently down to his left.
Przybek was in action again in the 17th minute, getting across to his left to bat Butler’s 25-yard strike, which was moving in the air, away from goal.
A minute later, the young keeper was forced into another impressive save, Ward hitting a top-corner-bound shot across him from the left which he again did well to palm away.
Aside from the occasional attack down the Blues’ right, all the threat had come from Posh. As the game approached the 20-minute mark Town wingers Roberts and Dobra swapped flanks.
And in the 23rd minute Roberts started the move which saw the Blues take the lead when he seized on a loose pass just inside the Peterborough half.
The ex-Crawley man fed Norwood, who took the ball on to the edge of the area before hitting a shot which Chapman saved but couldn’t hold on to and El Mizouni beat Roberts to the rebound to score his first senior goal.
On the half hour, Roberts hit a 30-yard freekick not too far over Chapman’s crossbar but a minute later the home side were level.
Town defenders backed off as 17-year-old Jones brought the ball into the area before rifling a low shot past Przybek to his left to claim his third goal of the season in only his fifth senior appearance.
In the 37th minute Tasdemir was shown the first yellow card of the night for a poor challenge on Dobra.
As half-time approached, Posh were pinning the Blues back in their area, Tasdemir hitting a shot which Henderson blocked, then Dembele struck an effort from distance which was diverted behind.
Posh would have felt hard done by to go in behind at the break, having looked the more dangerous side up to the Blues’ smash-and-grab goal and again the more threatening side once Jones had equalised.
For Town, Przybek could be pleased with his first 45 minutes of senior football having made two or three important saves, while Henderson also acquitted himself well after the early scare. Other than the goal and Roberts’s freekick, the Blues had rarely threatened.
The Blues will probably need to show more than they did in the first half after the break if they are to make it to round three.
Peterborough: Chapman, Mason (c), Bennett, Butler, Ward, Barker, Woodyard, Kanu, Tasdemir, Jones, Dembele. Subs: O'Malley, Cartwright, Kent, Reed, Burrows, Eisa, Toney.
Town: Przybek, Cotter, Henderson, Skuse (c), Kenlock, McGavin, Huws, El Mizouni, Dobra, Roberts, Norwood. Subs: Alley, Smith, Hughes, Gibbs, Simpson, Georgiou, Folami. Referee: Graham Salisbury (Lancashire).