Germany international Loris Karius suffered an injury blow in the pre-season friendly between Besiktas and Brescia.
The Besiktas keeper on loan from Liverpool has been ruled out for one-month.
Besiktas revealed that Karius suffered a muscle injury in his left leg while playing against the Italian side.
The 26-year-old keeper will miss the start of the Super Lig season which kicks off on 16 August.
Karius will not be in goal for the Black Eagles opener again Sivasspor on 17 August.
Besiktas have had a calamitous pre-season campaign on the injury front with nine players in total sidelined.
The Black-Whites head into the 2019-20 league campaign with a depleted squad.
The team have been struggling to adapt to the physical intensity demanded by newly appointed Abdullah Avci.
Avci resigned from his post as Basaksehir manager to replace Senol Gunes who took charge of the Turkey national team.
Avci guided Basaksehir to a runners-up finish in the Super Lig last term.