Sinckler had been among the most notable absentees when the squad was named last month
Ireland international Porter, 25, picked up the injury in Leinster’s PRO14 Rainbow Cup loss to Glasgow Warriors on Friday.
Sinckler was in the Lions squad for the 2017 tour of New Zealand but was among the notable absentees when the squad was announced last month.
“It’s really bad luck for Andrew and we send him our very best wishes for a quick and successful rehab,” Gatland said. “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.
“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.”
The Lions play Japan at Murrayfield on 26 June ahead of their July-August tour of South Africa where they will play eight matches, including three tests against the world-champion Springboks.