Kewell missed a penalty in the Heart's 2-1 loss to Sydney FC at Allianz Stadium on Sunday but Aloisi launched a passionate defence of his former Socceroos teammate after the Heart extended their winless streak to 15 matches.
"I'm disappointed in quite a few people questioning Harry, questioning his commitment," Aloisi said.
"People in general have been questioning 'is he here to play?' But I see him day in, day out work on and off the pitch.
"He's given everything possible to make sure that he can play and lead the team.
"Disrespecting someone like Harry and what he's done in the game, I don't know what people are thinking sometimes."
Down 2-0 at the break, the Heart had the perfect opportunity to get one back when they were awarded a penalty in the second half.
Kewell, who came on in the 56th minute, stepped up to the mark the 10 minutes later but sent his shot horribly wide.
Heart created a glut of other chances in the second half and Aloisi credited Kewell's presence for lifting the side.
"Credit to Harry he didn't drop his head he kept on going," he said.
"When Harry came on the whole team lifts.
"He's shattered because he missed the penalty he was saying sorry to everyone.
"But Harry will get through it because we need him to keep going and playing and he's working hard to make sure he gets it right."
Sunday was just Kewell's third appearance this season.
He was sidelined after the opening round with a neck injury and has since also suffered an ankle complaint.
He made his long-awaited comeback against Western Sydney last week and managed more than his 18 minutes in that game against the Sky Blues on Sunday.
Aloisi is confident he may even get a start in the Melbourne Derby next week.
"We have to see how he pulls up after today and see if he can have a full week in training this week," he said.
"Hopefully he'll start against Victory because he showed again his touches are brilliant."