With Switzerland maintaining their ideal record by winning 5-2 at home to Hungary, Portugal - who have confirmed a play-off place at least - will have to beat the Group B leaders in Lisbon to snatch top spot.
Tensions are reaching fever pitch for Portugal manager Fernando Santos, whose side sits three points below the Swiss with only two matches remaining.
Portugal will qualify as group winners if they beat Switzerland on Tuesday.
The 32-year-old came on for Gelson Martins at the start of the second half and 17 minutes later he did what all the best players seem to do - step up when their team needs them the most.
It took Ronaldo's goal tally to 15 in Portugal's World Cup 2018 qualifying campaign, with the Real Madrid star contributing to his side's second with a unsafe cross late on.
Ronaldo then provided the cross for Andre Silva's 86th-minute goal.
He spoke to RTP and said: "I thought [about it] a lot".
"I thought, well, Ronaldo did not train 100 per cent in two of the days of work".
The European champions were desperately short of inspiration as they struggled for more than an hour to break physical, ultra-defensive opponents on an artificial pitch that appeared to have taken a battering and was not in a fit state for a World Cup qualifier.
"I tried to administer the situation and do what was best. We won the game and that's what matters".
"We knew it was going to be hard and it was notable the team had difficulties adapting to the pitch, and then with their defence very close, [we were] lacking momentum". In the first half, we lacked momentum.
"I had to bring on Ronaldo at halftime and in the end, we won the way we wanted to".
Silva then added a second goal himself in the final stages, taking his career total to a hugely impressive 11 goals in 16 senior caps.