Sinckler, a breakout star on the tour of New Zealand four years ago, was devastated not to be selected in Warren Gatland’s initial 37-man squad, giving an emotive post-match interview to BT Sport after playing for his club Bristol Bears last month.
But the tighthead has received a call-up after Porter was forced to pull out with a toe injury sustained playing for Leinster against Glasgow on Friday.
Porter, who has 37 caps for Ireland but has not toured with the Lions before, was considered an important asset to the squad as he is capable of packing down on both sides of the scrum.
“It’s really bad luck for Andrew and we send him our very best wishes for a quick and successful rehab,” said Gatland. “Andrew is a young man with a big future ahead of him and I’m sure he’ll be in contention again in four years time.
Gatland said he hoped Sinckler “continues to stick two fingers up to me” about his initial omission.
“As always in rugby, when there’s an injury there’s an opportunity for someone else. I have been very pleased to see the way Kyle has responded to initially being left out of the touring party.
“He’s showed some excellent form of late and I hope he continues to stick two fingers up to me about leaving him out in the first place.”
Sinckler will battle with Ireland’s Tadhg Furlong and Scotland’s Zander Fagerson for a place in the starting XV against the Springboks’ fearsome front row.
The Lions face Japan at Murrayfield before departing for South Africa at the end of this month. Sinckler is unlikely to be available for that match, with Bristol well placed to make the Premiership final.
Sinckler is the 12th England player called into the squad.