The Blues captain has been hampered with a knee problem since the start of November and has made just 14 appearances for Rafa Benitez's side this season.
Cahill believes squad depth will be key in the final stages of the club's campaign and maintains he is prepared to compete for a regular role at Stamford Bridge.
"There will always be competition in a big club, with people fighting for spots and a massive number of games to play," Cahill told reporters.
"I needed to fight when I first came and that will continue now. You can never take anything for granted. The main thing is that it's positive for the squad that people are back fit now.
"John is a massive leader for us. He's been struggling with his injury but come back again. I'm repeating myself but it's great that everybody's back fit. It's a healthy situation."
Chelsea faces Sparta Prague in the second leg of their Europa League last-32 tie on Thursday evening, with Oscar's goal last week separating the two sides.
Cahill feels Benitez has a wealth of international-level defenders at his disposal and admits the Spaniard is right to rotate his squad.
"Different partnerships can work well depending on different games," he said.
"The lads have all played at the top level, all internationals, so it's good for the squad that we have faces back.
"We've had 20-odd consecutive midweek games, so that depth is important with David Luiz and JT [Terry] back for selection."