With Chelsea failing to reach the EFL Cup final after losing to Arsenal in the last four in midweek, Cahill is now hoping for a good run in the FA Cup.
"It was decent in the end," the Blues captain told BBC Sport. "I thought we started a bit rusty. We needed that win and to progress is the most important.
"We need to try to reach the final rounds in this having gone out of the EFL Cup the other night so this is very important.
"After the first 10-15 minutes we got on with it. Especially Michy, he took his opportunity today."
The draw for the fifth round of the FA Cup takes place on Monday evening.