The Cameroon international took time to settle after joining from Anzhi Makhachkala in the summer but has struck seven times in his last 10 appearances.
Cahill told the London Evening Standard: "Samuel has shown that he's world class in all his career. He's not lacking in confidence.
"Sometimes he's played, sometimes it's been Fernando [Torres], while Demba [Ba] has featured a little bit, but this hat-trick is going to do his confidence the world of good.
"He is a top, top player, a great performer. He's been involved in massive games in the past. It's great to have a player of that ability - with all his experience - in our squad.
"Will he be a massive asset in the title race? Yes. Samuel started this game and did fantastically well. He got the hat-trick which he thoroughly deserved. In the first half there wasn't too much in the game but we went in 2-0 up. He took his two chances and sometimes that's what makes the difference."
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