Real Sociedad have reached an agreement with Chelsea to take Sarr, 20, for the remainder of the season, according to Fabrizio Romano.
The defender joined Chelsea from sister club Strasbourg last summer, spent the autumn back out on loan at Strasbourg, and returned to Stamford Bridge in the spring, but managed only three league appearances for Xabi Alonso's side before this fresh loan was arranged.
More significant for Chelsea's longer-term planning is their growing interest in Ahanor, an 18-year-old defender regarded as one of the most promising talents in Italian football.
Romano reports that Chelsea have identified the Atalanta player as a name for the future, with the west London club tracking his progress closely.
Ahanor, who is contracted to Atalanta until the summer of 2028, has previously been linked with a host of Europe's biggest clubs, including Arsenal and Manchester City, underlining the scale of admiration for a teenager who only completed his move to Bergamo from Genoa a year ago.
Atalanta paid Genoa around €16million, plus a further €4million in add-ons, to sign Ahanor in 2025, and have already shown their reluctance to sell this summer by rejecting a €34million offer from Brighton.
Sporting director Cristiano Giuntoli has made clear the club has no intention of parting with its best young talents cheaply, meaning any move for Ahanor — from Chelsea or elsewhere — would likely require an offer well in excess of what Brighton had proposed.
Versatile enough to operate as a left-sided centre-back or a left wing-back, Ahanor made his senior debut for Italy this summer and has collected 28 Serie A appearances since his move from Genoa, form that has persuaded a growing number of Europe's leading clubs to monitor his situation closely as his stock continues to rise.
Chelsea's interest fits a familiar pattern for the club, whose recruitment strategy under BlueCo has consistently prioritised locking in elite young talent well before it reaches peak value, often via loan spells within the group's wider network of clubs.
Any move for Ahanor at this stage would represent an early step in a chase widely expected to intensify next summer, when Atalanta's resistance may prove harder to maintain if his form continues on its current trajectory.