"Kolarov, the new captain, and (Nemanja) Matic, his deputy, have the power to follow in Ivanovic's footsteps," Krstajic told a news conference ahead of warm-up games with Morocco on Friday and Nigeria on March 27.
"We have made a decision which remains an internal matter but I want to make it clear there is no misunderstanding with Ivanovic who remains part of the team," said Krstajic.
The experienced Ivanovic, who has scored 12 goals in 100 internationals and is one of only three survivors from the 2010 World Cup, has captained Serbia since 2012 when he took over from ex-Manchester United defender Nemanja Vidic.
During the qualifying campaign for the 2018 tournament, the 34-year-old former Chelsea player, now plying his trade with Russian side Zenit St Petersburg, was often criticised by fans and pundits, with many believing he has lost a yard of pace.
"Ivanovic has given a lot to Serbia and vice versa. But all players have to realise that they are starting the build-up to the World Cup with a clean slate and no one can take a spot in the 23-man squad for granted," added Krstajic.
Kolarov, 32, who plays for Italy's Champions League quarter-finalists Roma, has scored 10 goals in 72 games for Serbia.
The Balkan nation qualified for the 32-team World Cup after an eight-year absence from major tournaments, having made an early exit at the 2010 finals.
Ivanovic's demotion has come after the Serbian FA (FSS) sacked coach Slavoljub Muslin straight after he led the country to the World Cup and appointed the inexperienced Krstajic.
Muslin was criticised for what were perceived as unconvincing though productive performances in an unfamiliar 3-4-3 formation and for refusing to inject fresh blood into the team as he did not call upon several promising youngsters.
A former defender who played for Bundesliga clubs Werder Bremen and Schalke, Krstajic will make his competitive debut as a head coach at any level when Serbia take on Costa Rica in their Group E opener in Samara on June 17.
Brazil and Switzerland are in the same section and Manchester United midfielder Matic acknowledged that Serbia face an uphill task to reach the knockout stages.
"It's an extremely tough group," he said. "Brazil need no introduction or accolades and the other two teams are very strong as well. Right now we are focussed solely on our opening game against Costa Rica."
(Writing by Zoran Milosavljevic; Editing by Ken Ferris)
