The pope has said he would be ready to resign one day if serious health problems prevented him from running the Catholic Church.
Pope Francis said his advancing age and difficulty walking have ushered in a new, slower phase of his papacy.
“I don’t think I can continue doing trips with the same rhythm as before,” the 85-year-old told reporters aboard the plane returning to Rome from a week-long trip in Canada.
The pontiff had visited Canada to apologise for the Catholic Church’s role in schools where indigenous children were abused.
For the past few months, he has been using a wheelchair, cane, or walker because of knee pain caused by a fracture and inflamed ligament.
“I think that at my age and with this limitation I have to preserve myself a bit in order to be able to serve the Church, or decide to step aside,” Francis said.
He has previously said he could follow in the footsteps of Pope Benedict, who in 2013 became the first pope in 600 years to resign instead of rule for life.
“It’s not strange. It’s not a catastrophe. You can change the pope,” he said.
“The door is open. It is one of the normal options. Up until today, I did not use that door. I did not think it was necessary to think of this possibility but that does not mean that the day after tomorrow I don’t start thinking about it.”
He added: “This trip was a bit of a test. It is true that I can’t make trips in this condition. Maybe the style has to change, make fewer trips, make the trips I have promised to make, re-jig things. But it will be the Lord who decides. The door is open.”
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Pope apologises for generations of abuse
During his trip to Canada, the pope donned an indigenous feathered headdress before going on to say the forced assimilation of native peoples into Christian society destroyed their cultures and severed their families.
He apologised for Christian support of the “colonising mentality” of the times” and added: “With shame and unambiguously, I humbly beg forgiveness for the evil committed by so many Christians against the indigenous peoples.”
More than 150,000 native children in Canada were forced to attend state-funded Christian schools from the 19th century until the 1970s in an effort to isolate them from the influence of their homes and cultures in an effort to “Christianise” and assimilate them into mainstream society.