Striker signing Nunez, who scored on his Premier League debut against Fulham, has failed to add to his tally following a three-match ban for a red card against Crystal Palace in August.
But Sissoko – a Liverpool player for three years under Rafael Benitez – is confident the Uruguay forward simply needs time to adapt.
"It's not easy to play in the Premier League. It's a big difference between the Portuguese league and the Premier League," Sissoko said.
"If Liverpool spent a lot of money for this player, it's because he has quality. I'm sure he's going to score a lot of goals and show his talent, because he has talent."
Liverpool is down in eighth place in the Premier League after a disappointing start to the season in which it dropped points against Fulham, Palace, Manchester United and Everton.
And Nunez is not the only player to have struggled, with fellow forward Mohamed Salah criticised in scoring only twice in six league matches.
But Sissoko feels Salah's team-mates are as much to blame, saying: "It depends on the team also. When you play well, when you score, you make lots of good things, it depends on the team, not one player.
"The team has to play well, and after Mo Salah is going to show his talent."
With a tough run of fixtures ahead and with only two clean sheets so far, Liverpool will also need to improve defensively if they are to climb the table.
When asked about Liverpool's back line and recent scrutiny of Virgil van Dijk, Sissoko said: "He's still one of the best defenders in Europe.
"[Just] because he's not playing well in one game or three games, you can't say he's a poor player. He has quality.
"He's captain of the Netherlands national team, he plays for Liverpool, and he's shown everyone he's a good player. He's a leader also.
"Sometimes in football, [things] happen. Sometimes you play well, sometimes you play not good, but I'm sure after the international break all the team is going to win and [take] Liverpool higher."