The Bayern Munich forward missed his country's defeat to Brazil in their Group A opener on Thursday after being sent off in the second leg of their qualification play-off victory over Iceland.
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"Mandzukic definitely makes us stronger and you can't replace him like-for-like," the 32-year-old told Fifa's official website. "He's prepared to go out and die for the cause.
"Nikica Jelavic did really well against Brazil but Mario is Mario. We need him. We're a totally different team without him."
Kovac's side will be desperate to get off the mark following their 3-1 defeat against, suffered in controversial fashion with complaints that a number of key refereeing decisions went against them, and Pranjic warned his team-mates that it is now win-or-bust.
"It's all-or-nothing for both teams," he declared. The losers will pack their bags and go home.
"We have to be ready for everything and play like it was our last match."