Ipswich Town are through to the last 16 of the EFL Trophy after holding their nerve in a penalty shootout at Peterborough.
On a night where three young players, Adam Przybek, Alex Henderson and Tommy Smith, were handed debuts as part of a mix-and-match Ipswich side, the visitors won 6-5 on spot kicks following a spirited effort inside the 90 minutes.
Barry Cotter scored the winning penalty on his 21st birthday, while Przybek made two excellent saves on what was his senior debut.
Ipswich had been ahead, thanks to Idris El Mizouni's first-half goal, but his effort was cancelled out by Ricky-Jade Jones 10 minutes later and from there it seemed inevitable Peterborough would go on and claim the win.
Credit to Ipswich, though, as their young players held out and repelled the threat of Peterborough's 27-goal pairing of Ivan Toney and Mo Eisa after they were thrown on for the final seven minutes. The defence which finished the game, following the departure of Cole Skuse with a hip injury, had an average age of just 19 and did their job superbly.
The last 16 will be played in January, with Newport, Bristol Rovers, Portsmouth, Exeter, Stevenage, MK Dons and Walsall potential opponents. The draw is made tomorrow night at 7.15pm.
After Lambert had vowed to 'go strong' for this game, he named a side made up of a mix of young players and experienced pros.
While the likes of Cole Skuse, James Norwood and Emyr Huws were in from the start and formed a strong spine, there were debuts for goalkeeper Przybek and Under 18s captain Henderson in the centre of defence.
In total 11 academy players were named in the 18, with El Mizouni, Armando Dobra, Myles Kenlock and Brett McGavin all starting.
Henderson had it tough in the early seconds as Siriki Dembele stole the ball from him and ran towards goal before going down in the box, with the muted penalty appeals waved away by referee Graham Salisbury. Ricky-Jade Jones headed wide before Serhat Tasdemir was the next to take aim for the hosts, with the attacking midfielder lashing his shot over the top of the Ipswich bar.
Przybek had to get down well to hold Tasdemir's shot before Ipswich began to come into the game, with the direct running of El Mizouni and Dobra stretching the Peterborough defence down the right flank.
The young Ipswich goalkeeper remained in the thick of the action, with strong palms pushing drives from Dan Butler and Joe Ward away to safety as the Welshman quickly found his feet on his senior debut.
Peterborough were having the best of things, finding space between the Ipswich lines, but it was the young visitors who struck first.
A loose Posh pass was the root of it, as Niall Mason's ball into the middle of the pitch was seized upon by Roberts, who slid Norwood away. The striker took on the shot, left-footed, from the edge of the box but it was spilled by Aaron Chapman into the path of El Mizouni who reacted quickest to slot past the Peterborough goalkeeper.
Roberts, playing with confidence, took aim and fired a 25-yard free-kick marginally over the top, but within a couple of minutes the hosts were level.
Jones picked up the ball mid-way inside the Ipswich half and, as the away defence backed off, the homegrown striker ran towards the edge of the area and drilled a low shot past Przybek.
The two sides traded blows in midfield before the half-time whistle blew.
Skuse was replaced at the break, with Tommy Smith on for his debut at left-back and Kenlock moving to centre-half, with El Mizouni and Norwood then having tame shots easily saved by Chapman in the first minutes.
The hosts soon had a golden opportunity to go ahead, though, as Henderson got sucked towards Dembele's run, leaving Ward free at the far post to apply the finish. Thankfully for Ipswich his shot was wide.
By this point Peterborough were well on top and it seemed inevitable they would take the lead. Dembele was the next to shoot, firing wide of the Ipswich goal as the visitors struggled to get out of their own third of the pitch and, whenever they did, Norwood was unable to feed off the scraps he was given against three Posh centre-backs.
Tame efforts from Roberts and Huws came and went, as Ipswich managed to get something of a foothold in the final third, before Toney and Eisa were introduced from the bench.
Toney's first act was to foul Roberts on the edge of the box and earn himself a yellow card, with Huws then having the resulting free-kick tipped over the top by Aaron Chapman.
Huws and Chapman had one battle, with the Peterborough goalkeeper again the winner as he pushed the Welshman's shot away. The midfielder was in the thick of the action before the whistle blew for 90 minutes, superbly blocking Tasdemir's drive away.
Eisa and Roberts got their sides off to the perfect start in the penalty competition, held in front of the Peterborough fans, before Toney slotted his effort past Przybek.
McGavin was next up for Ipswich but saw his penalty saved by Chapman, before Przybek superbly kept Dembele's effort out and Kenlock netted.
Butler scored before El Mizouni's penalty snuck in, before Mason and Norwood both scored to tie things up at 4-4 and take it to sudden death.
Kyle Barker sent a rocket down the middle before Huws did the same, with Przybek keeping out Frankie Kent.
Then it was down to Cotter to net the winning penalty and take Ipswich through.
Peterborough United: Chapman; Mason, Bennett, Butler; Kanu (Toney 84), Ward (Kent 63), Barker, Woodyard, Tasdemir; Dembele, Jones (Eisa 84)
Subs: O'Malley, Reed, Cartwright, Burrows
Ipswich Town: Przybek; Cotter, Henderson, Skuse (c) (Smith, 46), Kenlock; McGavin, Huws, El Mizouni, Dobra, Norwood, Roberts
Subs: Alley, Hughes, Gibbs, Simpson, Georgiou, Folami
Attendance: 2,310
Penalties
P: Eisa - scored
I: Roberts - scored
P: Toney - scored
I: McGavin - saved
P: Dembele - saved
I: Kenlock - scored
P: Butler - scored
I: El Mizouni - scored
P: Mason - scored
I: Norwood - scored
P: Barker - scored
I: Huws - scored
P: Kent - saved
I: Cotter - scored
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