The striker missed chances to score during Chelsea's Europa League win against PAOK before being substituted with 10 minutes left.
Sarri was pleased with his team's victory over PAOK but admitted he was frustrated Chelsea failed to kill the game off after Willian's early goal.
Sarri introduced Olivier Giroud for Alvaro Morata in the 80th minute in search for more goals to seal the game.
"Alvaro has to gain confidence with one, two, three goals", Sarri added. In this match he has had three or four opportunities. "Of course there will be [a spot for him in the team] in the future". But the confidence can come with goals. For the first time in this season, he was ready in the box.
A radical departure from Antonio Conte's reactive approach, Sarri-ball has seen the Blues embark on an attacking rage at the league, plundering 14 goals so far, while also having made the most passes out of all the top-flight clubs and also having the best passing accuracy percentage (along with Manchester City).
Cahill is one of the few survivors from Chelsea's last Europa League game, when they won the 2013 final against Benfica after suffering the ignominy of dropping out of the Champions League when holders.
"I think we were in control of the match for 90 minutes". 'We had a lot of opportunities. I am very happy with the three points, but not the result.
Sunday's match against West Ham continues Chelsea's jam-packed start to the season, with a Carabao Cup clash against Liverpool on Wednesday before the two teams meet again at Stamford Bridge on September 29.