He managed to recover in time to play 90 minutes against Crystal Palace on the final day of the season but insisted he would continue working to regain full fitness and strength in his shoulder once the season concluded.
And the 19-year-old, who was on holiday last week, has been seen putting himself through a rigorous passing exercise on his own accord before the first team links up for pre-season next month.
Fosu-Mensah has now enjoyed almost 18 months in the first team but, with just 21 appearances altogether, remains somewhat on the periphery.
He has proved competent across the back four and actually prefers playing in central midfield above anything else, although a regular role in that department is highly unlikely.
The teenager will need to present himself as a reliable factotum for Mourinho next season, fitting into various roles when the Portuguese has to dig deep into his squad – as he did last year – and his early preparations will certainly pique the 54-year-old’s interest.