Maguire played every minute of every Premier League game of the season before coming off injured against Aston Villa a few weeks ago.
The defender has been unavailable since that game and despite that, Southgate picked up Maguire as one of his four centre-backs and Ferdinand does not think it is the right decision.
"England needed another centre back," he told his YouTube channel FIVE before adding, "This [squad] tells me he [Southgate] is definitely playing three at the back."
Ferdinand continued, "Harry Maguire's not even fit, I wouldn't even take Maguire. You're not fit, you're not going to be fit for ages, it doesn't make sense. I've been in squads with players who haven't been fit and they don't perform when they're not themselves."
He added, "It happened with Becks [David Beckham] and Wazza [Wayne Rooney]. As good as Maguire is, he's not a Becks or a Wazza. I don't think you should take someone who's not 100% fit. It's hard to come back and get into a rhythm."
The former English defender concluded, "That narrative ends up taking over the squad and it becomes the conversation and a distraction. Do we really need that? It's pointless I think. Hopefully he proves me wrong, I would love it if he did."