While our priority is a striker according to Ferran Martinez we're also looking for a left-footed CB (Umtiti replacement) who would be brought alongside Garcia.
Romagnoli (Milan), Lisandro Martinez (Ajax) and Marcos Senesi (Feyenoord) are "cheap" options the club likes and considers but there is no clear favorite among them so everything is open. Aissa Mandi was also considered although he's right-footed but could play as a LCB. His contract with Betis is expiring.
Don't know if he's the best CB among them but due to his versatility I'd prefer Lisandro Martinez.
If we'd really sign Garcia and another CB we'd have 6 CBs (not even counting FDJ who could also play there in 3 atb system) so all of this makes me think Koeman could still consider using 3 CBs at least sometimes next season. It's true all of them have question marks though. Pique is old, coming off serious injuries and should be phased out of the team. Araujo is good but can he stay healthy? Oscar is developing well but ideally shouldn't be a starter (not in a 4-man defense at least). Lenglet is Lenglet but unless we get a really good offer I'd keep him. He's durable and would bring some experience to other youngsters. I just think it would be naive going into next season with Pique and 3-4 kids. He should earn his minutes though (that stands for others too; with possibly so many CBs we'd have some serious competition, at least quantity wise if not quality-wise that much). And we don't know if/how/where Garcia and a possible another CB would fit into the team. Also can't forget we'd still need to find someone to take Umtiti (possibly on loan to just save some money on his wages) and unless Nice will use their buy option Todibo will also return and we'll need to find a buyer for him as well. At this point it's clear he has no future at the club so just sell him for whatever we can get.
Another option could be Jorge Cuenca. Barca has a buy-back option on him but Koeman apparently doesn't rate him so if he stays Cuenca probably won't return. TBH he was a starter ahead of Mingueza for Barca B and Oscar is doing well now in the first team so we can argue Cuenca could do the same. I'd still prefer to see how he'd do for Villarreal in La Liga next season (or maybe in Almeria if they're promoted and Villarreal would leave him there on loan for another season) before deciding to potentially get him back.