Eder Militao's season is over after suffering a second ACL injury in 18 months in yesterday's victory over Osasuna.
With Dani Carvajal and David Alaba also sidelined, Guti has raised the idea of bringing back former captain Ramos for the remainder of the season.
Ramos remains a free agent after leaving Sevilla over the summer, having made it clear he wants to play on.
Guti said on DAZN: "Look at (Carlo) Ancelotti's worried face. Militao is a very important player who had just recovered from last year's injury, who was getting back to his best and who is leaving Madrid's defence in a tight spot. He is joined by Carvajal's injury. Alaba has had a very long injury...
"You have to go out on the market. Sergio Ramos is without a team. Right back and centre back. In this situation, I clearly see him, at zero cost. He comes from playing last year in Sevilla and doing well.
"You can go to the market and if you really want to bring one in, paying a lot of money, knowing that he is a player for the next 10 years, go and bring him in. But in December you can bring in a patch if you don't know how things will go or if you will have him the following year. I think you have it clear: Sergio."