On the day two new Manchester City centre-backs flew to the Etihad for medicals, and another deal was agreed in attack, two existing defenders reminded transfer bosses of the quality simmering below the first team in that position.

Abdukodir Khusanov, 20, and Vitor Reis, 18, are en route to City to complete their moves for a combined £63m this month. Pep Guardiola wants a rebuild of his squad, clearly focussing on a defence where the key components are ageing and becoming less reliable with injury.

In the past 18 months, City have spent big on defensive reinforcements with Josko Gvardiol signed for £77m and earmarked for a role at the Etihad for the next decade. As City prepared for their FA Youth Cup clash on Thursday, reports emerged that another expensive defender is on the shopping list - 24-year-old Andrea Cambiaso. Then an agreement was reached to complete the signing of prolific striker Omar Marmoush.

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City don't hide from their youngsters the fact that breaking into the first team is practically impossible. It takes a generational talent (like Phil Foden), a well-timed breakthrough (like Rico Lewis) or sheer perseverence (like James McAtee) to do it successfully and even then there is no sentiment when it comes to external reinforcements. It's no coincidence that even players as talented as Cole Palmer, Romeo Lavia and Liam Delap are not at the Etihad.

So the teenagers on show in the Youth Cup won't be thinking for a second about following Foden and Lewis into the senior squad, even if captain for the night Kaden Braithwaite has started for Guardiola's men this season and Jahmai Simpson-Pusey has been fast-tracked for the seniors. The only target was reaching the fifth round.

On the other hand, Braithwaite will see a £50m left-back being targeted and wonder if that debut will be added to in future. Kian Noble - the impressive centre-back who stood out in last year's Youth Cup final - will wonder the same. So will 16-year-old Stephen Mfuni alongside him.

Mfuni turns 17 next month but might have the best chance of the lot in the under-18s. He scored in that Youth Cup final last summer and got his side off the mark against Millwall on Thursday night, creeping in at the back post to sweep home from a free kick. His interceptions were decisive, as were his passes through the lines, and he remains one of the more exciting talents in the academy.

Despite dominating the first half, City conceded a penalty as Millwall equalised from the spot, but Harrison Miles' deflected strike in injury time before the break restored the lead. Instead of folding again, Ryan McAidoo added brilliant solo effort for the third, Matty Warhurst converted a fourth on the hour mark before Divine Mukasa twisted and turned and scored a fifth soon after.

The first three scorers were all 16, while seven of last year's Youth Cup winners featured here, stepping up to take a more senior role as others have moved up themselves past the under-18s. It felt significant that three of the back four this season started the final in May while the other has a first team start to his name.

It isn't as simple as saying City shouldn't spend millions on young defenders like Khusanov, Reis or even Gvardiol. They have all proven themselves in first team environments in big leagues, and City's youngsters are not yet at that stage. City need reinforcements now, not in three or four years.

But with each expensive signing, the door closes a little more for a youngster to make his breakthrough in the future. They know it and it's up to them to prove the club wrong (or prove them right and recoup some of that money in the future).

For now, the future for these youngsters is continuing the charge in the Youth Cup. A repeat of last season's final looks likely in the next round and defending their title in May is the best way to prove themselves while the first team rebuild gets underway over at the Etihad.