Bilic told his pre-match news conference: "In the beginning of pre-season, Sakho got injured, came back in the game against Spurs away, he played a bit and in the next game we played Manchester United away.
"He did his hamstring, and was out for the operation and was out for four or five months, he came back and now he has got another problem. It's really a problem.
"He's not fit for tomorrow [Friday] and again it's his back and unfortunately he is out for the season. He's been very unlucky," the Croatian added.