Sturridge hit two goals against his former club Chelsea last week and was also on target in wins against West Ham and PSG.
The 29-year-old seemed destined to leave Anfield after an injury-plagued loan spell at relegated West Brom at the end of last season.
Team-mate Wijnaldum has revealed one thing that surprised him about Sturridge ahead of Liverpool’s match in Naples.
"I don't know what he did in his pre-season, but when he came back he was really fit,” Wijnaldum said.
Liverpool news: Wijnaldum reveals he was surprised by Sturridge (Image: GETTY)
Liverpool news: Sturridge was the hero at Stamford Bridge (Image: GETTY)
“Better than the years before. He's showed that he's really important for us, not only for his goals but also the way he's playing. He's really in good shape."
Sturridge is in serious contention to start the match with Mohamed Salah performing at nowhere near the levels he hit last season.
The Dutch midfielder says Liverpool must be wary of the Italians.
"They have a really good team. They have a lot of good players we have to be aware of. It's going to be a difficult game,” he added.
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Liverpool news: Sturridge endured a difficult loan spell with WBA (Image: GETTY)
Liverpool news: The Reds are still unbeaten in the Premier League (Image: GETTY)
"Every team has its weaknesses. It's not good to tell them in a press conference. We will try to use them to win this game. Play for a draw? We always play games to win. Tomorrow, we play the game to win it."
Klopp side sit top of Group C while Napoli are looking to respond from their disappointing draw against Red Star Belgrade in Serbia.
Carlo Ancelotti’s side were beaten last weekend by Juventus in Serie A and will be keen to make amends against the Reds.