What I'd say to that is:Everyone knows we are not replacing senior player straight with academy player. Even Foden had to bide his time and he is generational talent.
Bit over the top to say he MUST replace Mahrez, just because he plays well or scores good goal at U23 level.
IMO, academy players get far too much hype for too little.
Perfect example of this; When Foden was coming through all the attention were on Sancho and/or Diaz due to them being more flashy, even though they were much worse players.
Simply put, we ain't replacing former POTY of the PL with academy player unless he is literally next Foden.
1) Foden was 17. He was way too young to get valuable minutes, so he got some here and there (he only got 328 minutes). The next season when Foden was 18 he got 1107 minutes.
Palmer will turn 19 soon, so if he is involved next season he can't be getting the middling minutes Foden got in his first season here, it would have to be closer to Foden's second season, I don't expect him to get the minutes Foden got when Foden was 19, because as you said, Foden is special.
2) Foden had the benefit that Silva was the starter in the LCM role, and his backup Gundogan was rotating between all 3 midfield spots, so there was plenty of minutes there for Foden, especially in that 2nd season where you see he got 1107 minutes. His first season he wasn't a "real" squad player as ha barely played imo.
If you keep Mahrez, then Ferran is also playing there, so instantly you have 2 starters there that Foden didn't have to deal with. Combine that with the fact that Foden and Sterling can play there, that means there is no room for another academy player.
So... if the intent is to bring in someone like Palmer, then a RW has to go, doesn't matter who, at the end of the day there is no room there, Foden didn't have to deal with that. This is why I replied to the poster that Mahrez has to go for Palmer to get any meaningful minutes (>200 minutes).