The Sweden Under-21 international looks set to join Stoke on a temporary deal.
Guidetti, 21, joined City at the age of 16 but his progress has been hampered by illness and injuries.
He has appeared twice on the City bench this season but the club feel he would benefit from having greater opportunities elsewhere.
Guidetti is behind the likes of Sergio Aguero, Alvaro Negredo, Edin Dzeko and Stevan Jovetic at the Etihad Stadium.
He spent time on loan at Burnley just over three years ago and also had a successful spell at Feyenoord in the 2011-12 season.
Pellegrini, speaking at a press conference to preview his side's FA Cup third-round replay against Blackburn, said: "Yes, John is going on loan. I said two or three weeks ago.
"I don't know if he has arranged with Stoke but he is going on loan."
Stoke are first thought to be trying to convert midfielder Stephen Ireland's loan move from Aston Villa to a permanent deal.
That would then free up another loan space for Guidetti to move to the Britannia Stadium.
The Potters have Liverpool's Oussama Assaidi as their other permitted loanee.
Stoke boss Mark Hughes was asked about the possibility of signing Guidetti after his side's defeat by Liverpool on Sunday.
He said: "We are looking to try to add players if we can.
"There are certain parts of the game we need different options, to go in behind. We lack a little bit of pace and we are trying to address that."
- Sports & Recreation
- Stevan Jovetic
- Alvaro Negredo
- Sergio Aguero