Ngobeni, on loan at Stars from Mamelodi Sundowns, lost his place in the team after getting suspended for a clash with SuperSport United. He initially won a starting berth ahead of Johnny Mathole, who subsequently joined Royal Eagles in the January transfer window.
Mafoumbi, who missed the first half of the season through injury, kept two clean sheets against SuperSport and Golden Arrows, while he also impressed in the 2-1 loss to Maritzburg United at Harry Gwala Stadium this past weekend.
Letsoaka said: "You'll remember that against SuperSport he [Ngobeni] was suspended and Mafoumbi went on to play and now has two clean sheets.
"But how do you change a goalkeeper that has got two clean sheets? It is also to give him [Mafoumbi] the confidence to say, 'Thela must fight back for his position' and everybody is fighting for his position in the team.
"It's never a given that somebody owns the position in the team. Everybody's got to fight for his position."
Ea Lla Koto next host fellow strugglers Highlands Park on Sunday at Goble Park.