Son is at the Asian Cup too so they will need somebody to step up in his absence. I am doubtful Werner will come close to replicating the influence Son has.
Son and Werner are different strikers. He's not a one-to-one first team player for Son. Tottenham would therefore have to adapt their strategy, tactics and formation to Werner's style of play, which I can't imagine happening in the middle of the season. I'm really curious to see how this experiment will end. Tomorrow against ManU will probably be his first match.