Sarr featured for Strasbourg in their 2-1 defeat at home to Paris St‑Germain on Sunday night, which is likely to be his final match for the Alsace club.
Last January, Chelsea completed their first permanent signing from Strasbourg since the two clubs came under the same BlueCo ownership, in a £12m deal.
Sarr made one substitute appearance for Chelsea as they won the Club World Cup in the summer before returning to Strasbourg on loan in August.
Recently appointed Chelsea head coach Liam Rosenior described Sarr as “my son”, with “scary” potential to become “world class” during his spell in charge of Strasbourg last season.
The defender’s return, after winning the Africa Cup of Nations as a starter for Senegal in January, is likely to provoke anger among Strasbourg supporters.
Protests from a minority of fans earlier in the season intensified when Rosenior left the club to replace Enzo Maresca, who had fallen out with senior figures at Chelsea and departed on New Year’s Day.
Along with Fofana’s loan move, deals that see Sarr and Anselmino swap places are the 10th and 11th deals between the two clubs this season, including the one that saw Rosenior move for a compensation fee from France to England.
Chelsea‘s defence has not performed at the level of their attack in the Premier League this season and they rank only 11th for expected goals (xG) against despite having conceded the joint-third fewest goals.
Yet Chelsea can strengthen their defence in the short term by bringing back a player Rosenior likes working with, while they will re-double their efforts to sign a centre-back in the summer.
They are also interested in signing another midfielder and an attacker, having briefly expressed an interest in now-Aston Villa loanee Douglas Luiz and Manchester City winger Antoine Semenyo over the winter period.