Most recently, Matteo Politano seems like a pretty big regret. We've been scouring the face of the earth looking for a left-footed RW after the Malcom debacle (thanks for that, btw ) , and Inter just bought an academy graduate of ours from Sassuolo for 25m who would probably fit the bill quite nicely. I think we only got 3m from him. Probably not a star, but a capable starter on a Champions League side - and he's Roman.
Alessio Romagnoli is the one that hurts the most in recent times, though. Left-footed CB , likely going to be the national team starter for the next decade at LCB. We got a good fee for him (at the time) , but I think if Monchi had've been in charge at the time , there's no way we would've moved him. You can't give up Roman gems like that - especially to Milan. Roma fans rationalize it by saying he's a Lazio fan (he is) , but still. Jamie Carragher was an Everton fan , it didn't force Liverpool to boot him out.
If we're going back a bit further, Aquilani is probably the most romanticized loss. Roma was in serious financial trouble and we sent him to Liverpool , which I think ruined a really promising talent. He was never the same after that , and ended up being a journeymen.
Probably no one on the level of a Thiago , but still certainly regrettable decisions , imo.
I asked because that was always my disappointment with Roma. They seemed to produce some good young players but not all of them get much of a chance. I'm of the opinion that they don't all have to be starters and it's important to have homegrown players in the squad.
I didn't know Romagnoli was a Lazio supporter. I can understand some dismay at that. I was just struck at the time that a player of his potential and a left-footed CB to boot was let go. Even Bertolacci at the time was one who I would have expected to be kept as a squad player, but the money for him at the time looked good and of course he doesn't seem like such a bad sale at the moment.
I remember hearing about Politano a few years back, but don't think anyone knew much about him. Still sounds like the sort I'm talking about. Of course Aquilani is another one. Maybe not Thiago level, but he was quite good when he left.
At least you have Florenzi. My favorite Roma player and a great example for the younger guys to take note of. On a weird side note, I used to play Roma with Football Manager back in the day and I used to get Pastore before he went to PSG and Sanson and now they're both there, albeit rather late.