Sarr's adaptation to life in England has been helped by three key factors. First, he spent a month with the team during their pre-season in the United States, where Chelsea won the Club World Cup, allowing him to bond with his new team-mates.
Second, the appointment of Rosenior - whose footballing philosophy Sarr knows well - has encouraged a player once defined by physicality and height to add greater intensity and aggression to his game, both on and off the ball.
Those close to the young defender say he explored loan moves to Premier League and Champions League clubs in the summer following the Club World Cup, only for Rosenior to persuade him to return to France as Strasbourg vice‑captain while competing in the Uefa Conference League.
They add that, had Rosenior not replaced Enzo Maresca in January, Sarr had expected to remain at Stade de la Meinau until the summer.
Third, Senegal's Afcon triumph gave him the greatest moment of his career so far, having started the final as a replacement for the suspended Koulibaly.
Since agreeing his move to Chelsea, Sarr has taken two hour‑long English lessons every day and has made significant progress. He still spends much of his time with the club's French‑speaking players but also has his older brother living nearby in London.
His father Pape offers occasional advice but remains based in Lyon. He has been a steady influence, supportive without being overbearing.
"Our father never put pressure on him," Sarr's brother Abdou told BBC Sport. "In fact, it was more our mother who took him to matches and training.
"Our father stood back and let him develop his own love and determination for football. He is still the same now - he knows Mamadou is motivated enough on his own."
When Sarr returned to Stamford Bridge, he was presented with an 'Afcon champion '26' shirt by the squad - an emotional moment given that his father was part of the Senegal squad which lost the 2002 final on penalties to Cameroon, with Pape missing that game through suspension.
"It's his best memory in football," Abdou added. "His father couldn't play in that final, so it was emotional for all of us. Everyone is proud - and his father's pride is even greater, having grown up in Senegal and knowing how hard it is to win that trophy."
