Martinez has identified an area of the loan market that is growing in supply but has few qualified buyers.
Clubs such as Chelsea are hoovering up the contracts of promising young players, offering large wages and transfer fees to their parent clubs that bypass the more traditional "working your way up" system. However those clubs then have a problem of what to do with them. Even if they are willing to play the youngsters in the first team -which few managers actually will do, especially in the big games or with defenders when one mistake can easily cost an important goal - they can only fit in one or two.
They can loan them to much smaller clubs for the initial training, especially to get game time, but the final polishing is difficult. Most big teams regularly face opposition that sits back defensively and plays on the break and that requires different skills than playing for a smaller team that is doing the counter attacking or is facing a team of similar level. There is also the step up in pressure of playing in the big games and the fact that younger players often need far more coaching from the manager with careful handling emotionally and physically.
There are four managers in the premier league currently who have shown willingness to trust youth even in the high pressure games - Wenger, Rodgers, Pochettino and Martinez. The first two will be less attractive loanees as they will be direct rivals of the clubs doing the loaning and Southhampton seem to focus on their own academy.
I'd expect both Chelsea and City to be very open to considering loaning us young players next year plus Barcelona and several others of the big European sides. The key will be taking advantage of these offers whilst still rewarding our own young players and complimenting them with purchases of players (probably with a focus on experience)