The Belgium international lit up the top flight last season and was named PFA Player of the Year after spearheading Chelsea's march to the Premier League title.
However, Hazard has fallen well short of top form this term, as have Chelsea, who are 10th in the league and out of the Champions League after a last-16 defeat to Paris Saint-Germain on Wednesday.
Hazard's father, Thierry, suggested that a groin injury was the reason for his drop in standard – something that Hiddink agreed with.
"I think his father has made a fair judgement," Hiddink said ahead of Chelsea's FA Cup quarter-final against Everton at Goodison Park on Saturday. "He is not absent of injury.
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"It's an injury with which you can play, but if you are not fully, fully fit that also has an influence on your freshness. The worries have been there, it's true.
"Every now and then I see that he is capable of doing beautiful things and very efficient things. His father was very humble and made a very good comment about this.
"I have my concerns about him as well. Whether he should play when he's not top, top fit, or 60 minutes here or there."
Hazard was booed from sections of the crowd in Wednesday's 2-1 loss at home to PSG, which consigned Chelsea to a 4-2 aggregate defeat.
Some fans became irked by the forward swapping shirts with Angel Di Maria at half-time, but when asked if he had spoken to Hazard about the reaction, Hiddink responded: "Not yet, but I don't want to overvalue that."