The France-born midfielder joins Sydney from Czech First League club Mlada Boleslav. The 32-year-old, who has 25 caps for Senegal, also previously played in the German Bundesliga with Hamburg and in the UEFA Champions League with Czech side Slavia Prague.
"I like the ambition of the club and the head coach Graham Arnold, who I have spoken to," Tavares said. "I know he likes to play football with the ball on the floor, short, sharp and attacking football, and that is my sort of game which is ideal."
"He has played international football on numerous occasions and is definitely a signing to strengthen our team," Arnold added.
Abbas suffered a serious knee injury during an A-League game in November that is expected to sideline him for more than a year.