Khawaja, who replaced Travis Head in the middle order in the fourth Test, scored back-to-back centuries at the Sydney Cricket Ground in a stunning return after more than two years out of the Test side.
Head reclaims the number five position after contracting COVID-19 and being forced into isolation following Australia's series-sealing win in the third Test at the Melbourne Cricket Ground.
Harris scored a Test-high 76 in the low-scoring MCG match but has made a modest 179 runs at 29.83 for the series.
"I think he knew it was coming," Cummins told reporters. "The message to Harry is we think he's going really well.
"I thought Marcus has been really good, I think he's been growing each game. He's certainly part of the future."
Australia have yet to decide on their bowling attack for Hobart, with paceman Scott Boland under an injury cloud.
Jhye Richardson, who took a five-wicket innings haul in the second Test in Adelaide, could return to the side if Boland is ruled out.