Real Madrid are heading into uncharted waters in the 2021-22 season. That's because, for the first time in many years, Los Blancos won't have a settled, world-class centre-back partnership.With Sergio Ramos already at PSG and Raphael Varane joining Manchester United, Real Madrid have lost one of their best centre-back pairings of the last few years in one summer.Sergio Ramos says goodbye to Raphael Varane and their 10-year partnership at Real Madrid 🗣️ pic.twitter.com/fuOHH1Nqj4— B/R Football (@brfootball) July 29, 2021While Real Madrid did sign David Alaba in the summer, he is not an out-and-out centre-back. So the club need a specialist in that position.They only have two recognised centre-backs in Eder Militao and Nacho Fernandez. Victor Chust and Jesus Vallejo are touted to become fourth-choice centre-backs, which could prove to be a risky proposition in case of injuries to the first-choice options.On that note, here's a look at four reasons why Real Madrid should sign a new centre-back this summer:#4 Real Madrid lack a reliable figure at the backSergio Ramos' departure has left a gaping hole in Real Madrid's defence.The exit of Sergio Ramos from Real Madrid has left a gaping hole at the heart of their defence.Ramos was at the heart of Real Madrid's defence, thwarting opposition attackers for many years. He marshalled the back-line brilliantly, guiding and improving all other defenders around him.But the centre-back's departure to PSG has left Real Madrid without a reliable figure at the back. The likes of Nacho Fernandez and Eder Militao are error-prone, while youngsters Jesus Vallejo and Victor Chust are pretty inexperienced.Real Madrid have brought in David Alaba from Bayern Munich. But there are doubts whether he will be enough to tighten their defence after Raphael Varane also left Madrid to join Manchester United.Hence a new reliable centre-back should be an immediate target for Real Madrid in the transfer window.#3 Reliance on youngsters could prove costlyJesus Vallejo has been tipped to be Real Madrid's fourth-choice centre-back.Earlier this week, reports stated that Real Madrid could look to use either Jesus Vallejo or Victor Chust as the fourth centre-back in the first team next season instead of going after a new one.While promoting these youngsters is not a bad proposition, relying on them to fill the depth at the back could prove costly for Los Blancos.Vallejo, for example, has failed to impress on recent loan spells at Wolverhampton Wanderers and Granada. It has been years since he played effective football for Eintracht Frankfurt.So it is questionable whether Vallejo can perform at the required level when called upon. As was seen last season, with all the injuries in the Real Madrid camp, centre-back depth is not something to experiment with.Jesus Vallejo ready for Real Madrid first team role in 2021/22 https://t.co/NvhPeWrAvP— Football España (@footballespana_) July 12, 2021Meanwhile, Chust is a decent young player, but it does not look like he is ready to be a first-team centre-back next season just yet. Thus it would be risky for Real Madrid to rely on such young centre-backs, which is why they should look to acquire a more experienced centre-back in the summer.