Guardiola oversaw a one-sided 2-1 win when he and Mourinho resumed hostilities in September and was generally felt to have made a bigger impact than the Portuguese in the season's early months.
But having been condemned to a trophy-free first campaign at City by Sunday's FA Cup semi-final loss to Arsenal, Guardiola will lose his hold on a top-four place if United prevail at the Etihad Stadium.
"If we finish fifth and they finish sixth and we are above them, it means nothing," Mourinho said on Wednesday.
"If they finish third and we finish fourth, they are above us, but it means a lot.
"This is not about Pep, this is not about City; it is about fighting for objectives. We want to play Champions League football and we still have two doors open."
Mourinho was alluding to the fact United can also qualify for the Champions League by winning the Europa League, ahead of their two-legged semi-final against Celta Vigo next month.
But victory on Thursday would help to shut off City's only path to Europe's top club competition.
It would also raise the prospect of United finishing above City for the first time since 2013 and provide proof that Mourinho has made a quicker start to life in Manchester than Guardiola.
United have struggled for fluency this season, drawing no fewer than 12 games, and spent four months grimly locked in sixth place in the table.
As the days grow longer, however, the prospects of a successful end to the season appear to be getting brighter.
Their stunning 2-0 ambush of league leaders Chelsea showed they are a match for anyone and they are seeking to equal a club record by going 24 top-flight games unbeaten.
Fatigue aside, the only dark cloud on United's horizon is injuries.
Marcos Rojo and top scorer Zlatan Ibrahimovic have been ruled out for the rest of the season, while Paul Pogba will miss Thursday's game with a muscular problem sustained during Sunday's 2-0 win at Burnley.
Centre-backs Chris Smalling and Phil Jones remain sidelined, along with Juan Mata, who scored the winner when the teams met at Old Trafford in the League Cup fourth round in October.
Ibrahimovic's injury has opened the door for Marcus Rashford, who netted the game's only goal when United last visited the Etihad in March 2016.
Anthony Martial will also hope to start after scoring one goal and making the other for Wayne Rooney at Burnley.
Mourinho and Guardiola had a sulphurous rivalry during their time at the helms of Real Madrid and Barcelona, but Guardiola says their relationship is a courteous one.
"We are neighbours, but no," the Catalan said when asked if he and Mourinho had ever bumped into each other in Salford, where they both live.
"When we see each other, (it's) 'Hi, hi.'"
Guardiola says David Silva is a "real, real doubt" after the Spanish schemer sustaining a hamstring injury during City's 2-1 loss to Arsenal at Wembley.
But he had more positive news about Sergio Aguero, who trained on Wednesday after picking up a knock against Arsenal.
John Stones, Bacary Sagna and Fernandinho are also injury doubts, but January signing Gabriel Jesus could feature following his return from a broken foot.
Mourinho said it would be "naive" for any rival to think United will be unduly diminished by their injuries and Guardiola concurred.
"Ibra is a great player, but they also have lots of good players like Jesse Lingard, Ashley Young and Marcus Rashford and they find spaces between the lines," he said.
"They are good rivals, with good quality, in good form and have a big series without defeat in the Premier League. I'm looking forward to the game."