Santos, 22, joined Chelsea from Vasco da Gama back in January 2023, but did not get a real chance in the first team until last season, having spent time out on loan in the interim.
The Brazilian impressed when he was finally given his opportunity at Stamford Bridge, scoring one goal in 27 Premier League appearances, 13 of them starts.
Those performances have sparked interest from elsewhere in the Premier League, with Manchester United previously credited with an interest in the midfielder earlier in his time in England.
United last showed concrete interest in Santos back in the spring, but their attention appeared since then to shift primarily towards West Ham's Mateus Fernandes.
The Portuguese midfielder ended up joining Tottenham Hotspur instead, prompting United to look again at alternative targets in that area of the pitch this summer.
Santos is now said to be an option for Newcastle United as well, with the Magpies missing out on Fernandes as he completes his move to Tottenham.
The Brazilian is understood to be open to leaving Chelsea this summer in search of regular first-team football at another club, having established himself as a regular contributor last season after years of waiting for his chance.
Chelsea are said to be demanding £50million for the midfielder, reflecting his improved standing at the club.
Santos remains under contract at Stamford Bridge until the summer of 2030, giving the Londoners considerable leverage in any negotiations over his future this summer.
His situation is likely to remain one to watch closely as United and Newcastle continue weighing up their options in central midfield over the coming weeks.
Chelsea, for their part, will be reluctant to lose a player who showed genuine promise once given the chance to prove himself in the Premier League, and any deal for Santos is unlikely to come cheaply given his current contract situation.