Kostic was an unused substitute in Serbia's 2-0 defeat by the mighty Brazilians as there were questions over the Juventus player's fitness.
"The situation is not excellent, but I am satisfied compared to what we had before. The situation is better and that makes me happy," Stojkovic told reporters on Sunday.
"He has been training with the team for the last two sessions...I liked the intensity he worked at, which shows he is ready to play. I am glad he is fit and that we can count on him.
"I can't say if he will be playing but he is one of the potential players, which was not the case earlier. The way he runs and his physical engagement in training has shown that is going to be OK and that makes me happy."
Both Serbia and Cameroon lost their opening games, with the African side suffering a 1-0 defeat by Switzerland. The Serbians are languishing in last place in their group with Cameroon just above them on goal difference.
A defeat would knock Serbia out if Brazil fail to win against Switzerland, while Cameroon will be out if they lose and Switzerland fail to win against Brazil.
Stojkovic said the team struggled in their first game due to a number of injuries and fitness issues in the squad, including top scorer Aleksandar Mitrovic (28), but the situation is much better for Monday's game at the Al Janoub Stadium.
"It is an important game for both teams given that we suffered defeat in our first matches, but that does not diminish our ambitions and objectives of what we want to achieve," he added.
After Senegal defeated Qatar 3-1 and Morocco recorded a stunning 2-0 win over Belgium, Cameroon are aiming to be the third African team to win in Qatar.
Asked how Serbia will tame Cameroon, nicknamed The Indomitable Lions, Stojkovic said: "Cameroon is a team that can cause a lot of problems on the counter-attack so we have to be very careful tomorrow in order to prevent such a situation.
"We hope to be the team which will be on the top of their game and I hope that through our own style, we will be able to tame the Lions.
"Although it is not always easy because lions can be very fierce, but we have to play the best we can, I trust my team. I think everything will be perfectly fine."
'NO COMMENT'
Stojkovic declined to answer when asked about the impact of FIFA's investigation into Serbia's football association after a flag which showed Kosovo as part of their country was allegedly hung in the team's dressing room.
"No comment, next question," he answered.
The image of the flag, which was allegedly hung in the dressing room when Serbia faced Brazil, had the message "we do not surrender" in Serbian and went viral on social media when shared by Hajrulla Ceku, Kosovo's Minister of Culture, Youth and Sport.
Kosovo became a member of UEFA and FIFA in 2016, eight years after the country declared independence from Serbia, which opposed the move.
