Middlesbrough striker Tuncay will be sidelined for six weeks with a knee ligament injury he suffered on World Cup duty for Turkey last week.
The 26-year-old sustained a second medial ligament tear during the 1-1 with Belgium.
Middlesbrough manager Gareth Southgate told the club's website: "It is a real shame for Tuncay.
"However, we have a lot of strength in that area and his absence will provide opportunities for others in the squad."
Egypt international Mido partnered record signing Afonso Alves in attack on Saturday - and scored his fourth goal of the season - in the 2-1 defeat by Portsmouth.
The former Tottenham man had been at the centre of speculation he could leave the club during the summer transfer window with Wigan paying particular attention to his situation.
But Southgate said at the time Mido would get his chance, and he could be the man to replace Tuncay.
Meanwhile, midfielder Julio Arca returned to action after an ankle problem picked up in pre-season in Tuesday night's 2-2 reserve team draw with Hull.
The 27-year-old Argentine played the first half of the game, but might not be quite ready for Saturday's trip to former club Sunderland.
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