Sarr, who helped Senegal win the African Cup of Nations last month, was a key member of Rosenior’s Strasbourg defence during the first half of the season and captained the Ligue 1 side on several occasions during his time in France.

However, following Rosenior’s appointment as Chelsea head coach last month and Sarr’s return to west London on the final day of the winter transfer window, the two are now working together again.

‘Everyone knows what I think about Mamadou, I think he’s got the potential to be a world-class centre-back,’ replied Rosenior when asked for his opinion on the 20-year-old's return to the Blues.

‘I’m delighted he’s here and thank the sporting directors and ownership for supporting what I thought was the right decision for us and for him in his career.

‘I see a growth in Mamadou, even his performance in the AFCON final. I see a growth in his physicality, I see a growth in his tactical understanding. Also, it was an area of the pitch I felt we could strengthen.

‘He will get his opportunity, but he knows he needs to fight for it because we have got many outstanding centre-backs in our group.’

Rosenior also feels Sarr adds to the versatility which already exists in our back line, a quality he values highly in his players.

‘If you look at the flexibility of our defenders, Trevoh [Chalobah] can play all three positions, Wes [Fofana] can play on the right or centre, Mamadou can play all three, Josh [Acheampong] can play all three.

‘I like versatility and flexibility, in our build-up especially. I don’t say someone is "just a central centre-back or right or left centre-back", Mamadou can play every position and can do it at a very high level.’

It is not just in defence that our head coach looks for players to show the ability and willingness to operate in several different roles.

He points to Liam Delap taking up a wide right position in Tuesday’s Carabao Cup semi-final trip to Arsenal with a specific purpose in mind.

‘We really wanted to match Arsenal’s physicality at home,’ said Rosenior. 'If you look at [Piero] Hincapie, the two centre-backs, [Jurrien] Timber or whoever plays, they’re beasts and they’re strong. So, we wanted to be able to mix it up with them physically.

‘I’ve worked with Liam before and he’s played for me on the right and been outstanding there. His position is a No.9, but I like to use players in different positions.

‘I’ve used Garna (Alejandro Garnacho) as a second pocket, I’ve used Pedro (Neto) as a winger in the pocket, Cole (Palmer) as a wide player in the pocket. I like to have players who can play different roles.

‘Also, we didn’t have many players available. Everyone says it was a decision, but of our wingers Jamie Gittens was out, Pedro Neto wasn’t available, Garna mainly operates off the left, and Estevao had been in Brazil for the correct reason.

‘There are so many things that go into it and I thought Liam acquitted himself really well in that game.’