Everton striker is set to have surgery next week after injuring his anterior cruciate ligament (ACL) while on loan with Crystal Palace.
The 28-year-old was due to spend the rest of the 2019/20 term at Selhurst Park after completing a temporary switch to Roy Hodgson's side in the January transfer window.
However, after making five appearances for his loan side and scoring one goal, the striker was hurt in a training session on Monday and returned to Everton to be assessed this week.
The Blues confirmed on Thursday that Tosun had injured his ACL and will undergo surgery next week.
The player will continue his rehabilitation under the care of Everton's medical staff at USM Finch Farm following the procedure.
There is no set time-frame yet for the player's return.
Tosun has made 51 appearances for the Blues since his move from Besiktas in January 2018 scoring ten goals in that time, with his single strike for the club this term coming in November's 1-1 draw with Spurs.
