The 25-year-old Briton was called upon after Malukas recently sustained a wrist injury while mountain biking that required surgery last week and will leave him out for at least six weeks.
Ilott spent the past two seasons with Juncos Hollinger Racing, scoring a best finish of fifth in 2023 at St Petersburg and Laguna Seca.
Left without a drive for the upcoming IndyCar campaign, he switched to the World Endurance Championship to drive a Porsche 963 for Jota in the Hypercar category.
PLUS: The apprenticeship that will aid Ilott's IndyCar to WEC switch at Jota
“Obviously I feel sorry for David and the team,” Ilott said.
“It’s not easy to be in this position especially just ahead of the start of the season. I wish him the best and a speedy recovery.
“I’m glad I’m able to help out the team, and it’s a privilege to test this week with Arrow McLaren. I’m thankful they’re considering me for the opportunity.”
Pato O’Ward will be in the car on Tuesday, Ilott will test with the team on Wednesday, before Alexander Rossi rounds out the three-day hybrid test on Thursday.
The team is currently considering options to replace Malukas aboard its #6 Arrow McLaren Chevrolet entry for the season-opening Grand Prix of St. Petersburg and will announce who will pilot its third car in due course.
Ilott's WEC commitments do not clash with the event on March 8-10, one week after the WEC's opening round in Qatar.
“We all wish David a speedy recovery, and we’re working out some options with the No. 6 NTT DATA Arrow McLaren Chevrolet crew in the meantime,” said Arrow McLaren team principal Gavin Ward.
“Callum had a seat fit last week and will test with us in Homestead, and we’re looking forward to working with him this week.
“Then, hopefully we’ll be able to confirm our roster for St. Pete soon after.”
