O'Neill was fired after Sunderland were dragged into trouble at the wrong end of the table and volatile former Swindon boss Di Canio has been tasked with keeping the Black Cats in the top flight.
The Italian's first match in charge was at Chelsea on Sunday and, having lost 2-1 after leading 1-0 at the break, it is now only goal difference that is keeping Sunderland out of the drop zone.
And Di Canio suggested afterwards that one of the problems he needs to address is the fitness of his squad.
"I'm not going to say they're not fit, but it's not the fittest team in the world," he said.
"This team at this moment is not a team that can play Chelsea for 95 minutes.
"But we are going to work and give them more energy in the next few days and weeks. You can't do it in the way you want, but we're obliged to give more energy.
"We've studied 24 hours a day, how to give them something more without giving them problems or causing damage."
However, Di Canio remains confident of survival and added: "I'm not worried, I'm a very good manager.
"My new team, we've only had a few days together, but I've had good answers from them.
"Some of them have told me that what we've done in the last few days has been fantastic. Technically and tactically, they've said that's what we needed."