Mandaric had initially decided to let Nikolic keep his job after the 36-year old Serb called Nigerian striker Blessing Eleke a "black idiot" during Olimpija's 1-1 home draw with Zavrc on April 10. Mandaric reversed his decision following the ban.
"It was not an easy decision but as the club's president I had no other options," former Leicester City and Portsmouth owner Mandaric told a news conference.
"Nikolic is a good coach but he committed an intolerable offence, while Olimpija, a club that I took over because of its huge potential, has to remain a role model of professional conduct," he said.
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Nikolic insulted Eleke after the striker's exuberant celebrations of his stoppage-time equaliser against Zavrc meant league leaders Olimpija had less time to find a winner.
The coach apologised to the player after the game and Mandaric said he had refrained from firing Nikolic to give "a talented coach with a bright future" a second chance.
However, Mandaric changed his mind after the Slovenian Football Association's punishment, which meant that Nikolic would have been kept away from the dugout until the end of the season.
Olimpija top Slovenia's 10-team first division with 62 points from 30 games, two more than champions Maribor with six games remaining.
(Writing by Zoran Milosavljevic in Belgrade; Editing by Gareth Jones)
