On as a 32nd-minute substitute for the injured Christian Saydee, he produced a brilliant finish on 77 minutes after superb work from Gavin Whyte.
John Mousinho’s men found it tough to dominate possession against a Peterborough side who had won six on the bounce in all competition.
Yet Yengi and his team-mates found a way – and here are our Pompey player ratings...
1. Will Norris - 7
What a return to Peterborough. One good save in the first half, yet generally was asked to deal with high balls and mop up at the back. That’s now 18 clean sheets in the league this season. Photo: Jason Brown
2. Zak Swanson - 7
First start since December and handed the unenviable task of silencing Mason-Clarke. Stood off him a couple of times in the first half, but generally did well and can be proud. A few useful runs forward too. Photo: Jason Brown
3. Sean Raggett - 8
Didn’t give an inch, never lost a tackle or header. An absolute rock and also used the ball well. Such a warrior for Pompey. Photo: Jason Brown
4. Conor Shaughnessy - 8
So cultured, so calm, so good on the ball - and such an excellent defender. Marshalled Posh’s attacking threats magnificently, didn’t put a foot wrong. Photo: Jason Brown