The 30-year-old joined the Citizens on a free transfer from Arminia Bielefeld last summer and has since acted as a back-up option behind first-choice shot-stopper Ederson.
Ortega was handed only 14 started between the sticks across all competitions for the treble winners last season, including three Premier League outings and two appearances in the Champions League.
Nevertheless, the German was an integral part of Man City's success in the FA Cup, keeping five clean sheets in six starts.
Ortega started for the Citizens in their Community Shield defeat on penalties to Arsenal earlier this month, before being replaced by Ederson for the 3-0 Premier League win against Burnley last Friday.
Ortega, who has two years remaining on his contract at the Etihad Stadium, has recently emerged as a potential target for Bayern Munich, who have allegedly held talks with the player's representatives.
The perennial Bundesliga winners are keen to recruit a new goalkeeper following the sale of Yann Sommer to Inter Milan and the injury concerns over 37-year-old Manuel Neuer, who is recovering from a broken leg sustained last December.
Although Ortega is regarded as a second-choice option at Man City, Guardiola is keen for the German to stay at the club, especially with just a few weeks of the summer transfer window remaining.
Ortega is expected to start for the Citizens in Wednesday's UEFA Super Cup clash with Sevilla in Athens, and Guardiola has responded to questions surrounding the future of the shot-stopper.
Speaking at a press conference on Tuesday, the Catalan boss said: "It would be a problem to lose [Ortega] first of all because of the quality.
"We bring him here because (head of goalkeeping) Xabi Mancisidor sent me the clips and said 'this is a keeper that maybe we need'.
"I didn't know him. He was playing for a side relegated from the Bundesliga and I saw him and said 'ah, looks good'. But since day one I said 'what a keeper we have'.
"You know how many important games he played and how he saved us. We don't want to sell him, we don't want to loan him. We need him.
"But like many cases it depends on three parts because after that if we lose Stefan two weeks out [from the end of] the market, we have to go pick up a keeper and it's not easy now.
"You have to pay transfer [fee] and it's not easy. We want him. I am sorry but we want him."
Man City's third-choice goalkeeping option is 37-year-old Scott Carson, who has made just two senior appearances for the Citizens since joining the club in 2019.
Meanwhile, Guardiola has confirmed that playmaker Kevin De Bruyne is set to undergo surgery on a hamstring injury and could be sidelined for up to four months.