The Englishman has had surgery after rupturing his Achilles tendon during a friendly in Boston last week, ruling him out of the Europa League final.
“The conditions in Boston were not very good but there is not only one cause for the injury,” said Sarri.
“It is a similar injury to what Callum Hudson-Odoi [is suffering with] and the recovery time is almost the same.”
Both club academy graduates will miss next week’s European final with Arsenal in Azerbaijan, as well as sitting out England’s Nations League semi-final with the Netherlands on 6 June.
Speaking after naming his squad for the Three Lions’ Nations League finals campaign, manager Gareth Southgate said: “Callum and Ruben would definitely have been in this squad, so it is unfortunate for both of them that they have picked up injuries which may keep them out for some time.”