Rosenqvist was involved in a brutal crash in the first of two races at Detroit last weekend and has still not received the necessary medical clearance to return to the grid.
That's led his team to turn to Magnussen, who previously drove for McLaren in Formula 1.
The Dane in fact scored a podium on F1 debut for McLaren at the 2014 Australian Grand Prix and went on to race another five seasons in the world championship – one with Renault and four with Haas.
With Haas opting for a double rookie line-up this year as it rebuilds for the 2022 F1 season, Magnussen was set free to explore other categories, including racing in the American IMSA SportsCar Championship for Chip Ganassi Racing.
When the opportunity arose to try his hand at IndyCar this weekend at Road America, it proved too good to refuse.
“Making my IndyCar debut this weekend in the Arrow McLaren SP!” Magnussen, 28, wrote on social media.
“Standing in for [Felix Rosenqvist] while he recovers from his crash last week.
“Big challenge to go straight into a race having never driven an IndyCar before but I couldn't say no to the opportunity. Get well soon Felix.”
That followed a statement from McLaren SP: “Following an evaluation by the IndyCar Medical Team, Felix Rosenqvist has not been cleared to race at this weekend's REV Group Grand Prix at Road America.
“Felix will continue to be supported by the team as he progresses in his recovery…
“Felix will be replaced for the REV Group Grand Prix by Kevin Magnussen, who will be making his IndyCar debut. Magnussen made 19 starts for the McLaren Formula 1 team in 2014, including a career-best finish of second at the Australian Grand Prix that year.
“Further updates about Felix's status will be shared in due course.”
Magnussen will join long-time former Haas F1 team-mate Romain Grosjean on the Road America grid, the latter having taken a podium at the Indy Grand Prix last month in just his third start for Dale Coyne Racing with Rick Ware Racing.
Magnussen will be one of two substitute drivers competing this weekend, with Oliver Askew already confirmed to stand in for Rinus VeeKay after the Ed Carpenter Racing star suffered a fractured collarbone in a cycling accident this week.
Askew had filled in for Rosenqvist in the Sunday leg of the Dual in Detroit.
McLaren SP's Pato O'Ward leads the standings at the halfway point of the season.