Regardless of formation, we can't keep the ball. Our biggest problem is decision making with the ball and general ball retension. Contrary to some opinions, Gana is good at keeping the ball. We are only AVERAGE at winning the ball back, because Gana is literally the best at it in the league and one of the best on the planet. So buying players that make good decisions and can control the ball would be ideal. At the DM position AND frankly, the CM position, we need to have more athleticism and defensive either positioning or ability to win the ball back because without Gana - well, we say the Tottenham and Man City games. We lose Gana we may become the WORST at getting the ball back in the PL. The other teams can do basically whatever they way with the ball. It'd be worse than when Barry played and the other team could just do whatever they wanted in central midfield. Mix that in with a team that already can't take care of the ball and we'd just get smoked every week.
Silva is using his 4-3-3, but he is certainly adapting to his personnel as best he can. He has almost no CM/DM options, so he's pretty limited tactically. He said from day one that his preferred system varies depending on the type of player he has in his #6 role. Now he specifically cited Morgan Schneiderlin as his anchor man type (think Carvalho at Sporting), however I think Silva has realized pretty quickly that he isn't good enough. He has played 5 in the back at one point, but that's about it. It's been Siggy at 10, Gomes at 8, and Gana at 6.
Ideally, with this team's inability to control and keep the ball when attacking with width, Silva would I THINK rather play with 2 DMs/CMs behind a pivot/distributor that maybe sits a bit deeper than a 10, but can get into the box and score. Siggy absolutely can play that position. It doesn't require ball retention, it requires quick distribution to the wide forwards or #9, assuming they can hold the ball (they currently can't). Gomes could probably play that position. But Gana is literally the only DM we have. Beni maybe, McCarthy (maybe / doubtful), and definitely not Morgan (way too unathletic) or Davies (positionally clueless). So to me, Siggy isn't the issue with that formation, the lack of DMs is.
Wilmar Barrios would be the type of 6 in that 4-3-3 (really a 4-2-1-3), but that's a big jump in level. Gana sitting next to him would be a change, but if he goes...I mean, we are hosed. Just hosed. We'd need 2 CMs/DMs and better hope that they can collectively improve our ability to control the ball and not completely leave us with NO ability to win the ball back.