The victory, within four days, was just reward for skipper Kohli's insistence on playing five bowlers. Kohli led the way with a superb 200.
We did not use was just a reward because was just reward has an idiomatic usage.What do you say?
The meaning of "just" here is adjectival: "well-merited", "well-earned". In short, he earned the reward, therefore the reward was "just".
I also half-expected an indefinite article there, but in another position:
The victory, within four days, was a just reward for skipper Kohli's insistence on playing five bowlers. Kohli led the way with a superb 200.
However, it seems like "reward" can be used as an uncountable noun, in which case there's no need for the article.
Indeed, the Oxford Learner's Dictionary indicates that the noun can be used as [countable, uncountable], and quotes an example:
Winning the match was just reward for the effort the team had made.
This usage of "just reward" is in parallel to the expression "just deserts."
"Just deserts" means "getting the comeuppance you rightfully deserve."
Losing his business was just deserts for his unethical practices.
"Just rewards" is the complement -- getting the reward you rightfully deserve.
In both cases, "just" is used to refer to "justice."
"Just a reward" means "merely a reward."
The money was just a reward; the real payoff was the satisfaction.
I always used the spelling "just desserts," but I was corrected by Wiki.