Sissoko has been out of action since November 8, with a dislocated shoulder.
Riise said: “Before he got injured he was maybe our best player. He is so hard to play against because he never stops running and never stops being at your heels.
“The only thing missing is him getting a goal! We all hope he gets one soon. The amount of work he does for the team is very important for us. If he’s back then it will boost the team a lot.”