The Brazilian international joined the Reds in the summer and made his debut under then-head coach Steve Cooper a few days later in a Carabao Cup defeat to Burnley FC.
A Premier League debut followed at the end of October, when the 19-year-old was introduced from the substitutes’ bench against Liverpool FC at Anfield.
Santos didn’t feature under new Nottingham Forest head coach Nuno Espirito Santo and returns to Stamford Bridge having made two appearances during his time at the City Ground.