The 26-year-old Frenchman's initial loan deal expired after Saturday's 3-1 defeat at Fleetwood, where he was made captain.
But the clubs are in negotiations over extending his stay at Fratton Park and a deal should be finalised this week.
"There's been very positive talks so it should be sorted in the next couple of days," Diagouraga told The News.
"I'm happy to stay and it looks like Brentford are happy for me to stay as well.
"At first I was going back, then not. I just want to play football at the end of the day.
"If Portsmouth want me to play football here the best place to be is here."