Los Blancos endured a nightmare 2018-19 season, finishing third in La Liga - 19 points behind winners Barcelona and eight points adrift of city rivals Atletico Madrid in second - were knocked out of the Copa del Rey at the semi-final stage by arch-rivals Barcelona and were dumped out of the Champions League at the last 16 stage by Ajax.
In that time, they relieved both Julen Lopetegui and Santiago Solari of their managerial duties and reinstated Zidane, who had left that summer and is now primed to lead a drastic turnover in playing staff.
Left-back was among the many positions in need of improvement, and while Marcelo has had a year of turbulent form, Zidane appears set to keep faith with him.
However, he does not appear to believe that Reguilon, Theo Hernandez or Achraf Hakimi - whose loan at Borussia Dortmund has been extended - are of the requisite quality to compete with and eventually replace the Brazilian, leading him to search for a replacement.
Mendy has enjoyed another impressive campaign with Lyon, demonstrating his speed and technique in his 43 appearances across all competitions in France this term.
He will turn 24 in June and, with Marcelo the wrong side of 30, is an excellent candidate to fill the Brazilian's boots for years to come.
Mendy has not hidden his desire to join Zidane's squad and a transfer could happen in the first few weeks of the window.
However, he under contract with Lyon through 2023 and should the Frenchman prove too expensive, there are other alternatives for Los Blancos.
Junior Firpo has produced a stellar campaign with Real Betis - scoring and assisting in a memorable 4-3 victory over Barcelona - and could be a cheaper option.
Alternatively, Reguilon could be retained for another season if the right deal cannot be struck.
Elsewhere in defence, Eder Militao has already agreed to join, which will likely see Jesus Vallejo and Nacho Fernandez leave the club.
Meanwhile, goalkeeper Keylor Navas will likely join a raft of midfielders and forwards in exiting the Spanish capital.