Why would we loan the potential young GK if we need him now? Liverpool has Alisson, Chelsea had Cech when they loaned Courtios/Mamardasvhili, we will have MATS who was bad before the horrible injury and probably 35 year old Szczesny. A good young GK can become a starter in no time if judged fairly.
The key is to offload MATS and buy a young GK with great potential and play him while having Szczesny as backup. He already said he's willing to help from the bench. Knowing our club there is 99% chance that we will go into next season with MATS/Szczesny combo with MATS walking back into the starting lineup. We never learn...
Because he needs to play regularly.
I just don't see your scenario where we push MATS out next summer (especially after injury), spend big on a young GK who would start immediately and keep Szcz as a backup (I think he'll go back to retirement as soon as he'll think he's not a guaranteed starter anymore) very likely.
Like it or not MATS is not going anywhere for at least two if not three years.
But ofc it depends on who a choosen GK will be, what his current level is and how much we would pay for him.
I don't think we are in a position to go for expensive ones like Costa or Verbruggen who are both in the 50m range (whether that's a fair price or not is another question). That would be a clear message for MATS to GTFO but very unlikely to happen.
So we are probably looking at 15-20m goalies like Bulka, Carnesechi or Messlier (all in their mid-twenties and with contracts up in 2026 so that's probably a realistic value). It's up to our scouts to decide who amongst them is the best fit for us if we're looking at that tier. I think they would all agree to come in and "compete" for minutes in first season and then take over. Is any of them really good enough to become a long term starter? I'm not sure. But they would provide the competition (and injury guarantee for MATS) that I think the club is looking for like
@khaled_a_d said.
Other alternatives are pending free agents like Meret, Walter Benitez and Valles (I'm 99,9% he'll return to Betis otherwise he would be my prefered signing) but they won't be long term solutions either. In other words I see them more as replacements for Szcz if he'll go back to retirement after this season (which I still believe to be very likely) to be a competition for MATS rather than his longterm replacements. They would be clear upgrades over Pena but would be disappointing if that's the direction Deco will decide to go. Wouldn't be totally surprised though as we'll have other priorities next summer.
And then we are in a high risk/potential high reward territory with some of the names mentioned in previous posts like Restes, Suzuki, Kaua Santos,.. Will we really sign one of them? Probably not. But those are GKs that I meant as potential loanee candidates or more likely future arranged deals (so sign them next year and then they join in 2026). I think all of them would benefit more playing regularly for their current teams than spend most of the first season on the bench here.
BTW, I've seen Lucas Chevalier linked to Madrid recently. Should definitely be considered too.