The Spaniard joined Man City from Girona in 2019 in a £11million deal but is yet to register a single appearance for the club.
Porro went straight to Real Valladolid on a season-long loan, featuring 15 times as he helped them to 13th place in LaLiga.
He later joined Sporting Lisbon on a two-year loan deal and delivered the league and domestic cup double during his first campaign.
UEFA rules don't prohibit players from featuring against their parent clubs as is the case in the Premier League.
Which is why there are so many cases of footballers haunting their current employers including Philippe Coutinho's infamous brace against Barcelona.
Real Madrid conceded a goal to James Rodriguez when he was on loan at Bayern Munich in a 2-2 draw in 2018 while Thibaut Courtois helped Atletico Madrid eliminate Chelsea four years prior.
City even have history of their own players hurting them in the Champions League following Patrick Roberts' goal for Celtic in a 1-1 draw in 2016.
And Porro - who has scored three goals in 13 league games this term - appears to harbour no warm feelings towards City.
The Tottenham target declared last summer that Guardiola has never spoken to him since his arrival at the Etihad.
Laughing, he told A Bola: "No, I never spoke to Pep Guardiola.
"I don't think he even knows they hired me. But they must be attentive to what I have been doing."
Guardiola did not take too kindly to Porro's comments when he was asked about the Spain international ahead of the last-16 first leg.
"I know him perfectly but we have 250 loan players and I cannot speak with them all," Guardiola said.
"I'm happy it's going well and of course I know him. I saw many games and sometimes he played and sometimes he doesn't."