The Serbian FA were fined after fans lit flares and clashed with security stewards in the March fixture between the two sides in Constanta and Jovanovic said any repetition of such behaviour would harm the team.
"We want our fans to get behind us without causing any trouble," he told a news conference in the team's Kovilovo training camp near the Serbian capital.
"They need to understand that they will only help us if they don't let their tempers get the better of their common sense and we hope that they will create a positive atmosphere."
Serbia top the group with 19 points from eight matches and need one win from their last two qualifiers, against Romania or in Lithuania next Wednesday, to seal a berth in next year's finals in South Africa.
Romania have only pride to play for after a poor campaign but Jovanovic believes lack of pressure will make the new-look Romanian side more dangerous.
"The Romanians are rebuilding and will be completely relaxed, while we have to try to finish the job on home soil.
"We are confident that we are the better team but we have to prove that on Saturday and earn the win we need to make the final step towards South Africa.
PENULTIMATE EXAM
"Our previous games were just as important because it was the impressive run that got us here but this one is specific because it is our penultimate exam and victory would make the final one a formality," said Jovanovic.
"It would be a great honour to play in the World Cup because so many good players have never played in the finals and it could be the chance of a lifetime," said in-form midfielder Milos Krasic.
Serbia are likely to start in a 4-3-2-1 formation, with Krasic and Jovanovic providing attacking support on either flank behind towering hitman Nikola Zigic.
Romania will welcome back striker Adrian Mutu, who will make his first appearance under coach Razvan Lucescu after missing several games with a thigh injury.
Lucescu, who is undefeated in four matches since he took over from Victor Piturca in April, is likely to reshuffle his starting line-up again as he rebuilds the side.
"I would like to have a starting line-up as consistent as Serbia's but we are still at a stage when we are trying to build a core for the future," he said.
Teams (probable):
Serbia: Vladimir Stojkovic; Ivan Obradovic, Aleksandar Lukovic, Nemanja Vidic, Branislav Ivanovic; Nenad Milijas, Gojko Kacar, Dejan Stankovic; Milan Jovanovic, Milos Krasic; Nikola Zigic.
Romania: Danut Coman; Vasile Maftei, Mirel Radoi, Cristian Chivu, Razvan Rat; Dacian Varga, Tiberiu Ghioane, Iulian Apostol, Bogdan Mara; Adrian Mutu, Ciprian Marica.
