Magnussen was dropped by Haas at the end of the 2020 season and spent last season in sportscar racing, contesting the full IMSA WeatherTech SportsCar Championship with Chip Ganassi Racing prior to signing a contract with Peugeot's nascent Le Mans Hypercar project.
But the Dane was recalled by Haas to replace the ousted Nikita Mazepin on the eve of the second F1 test in Bahrain and followed up the team's first Q3 appearance since 2019 with a remarkable run to fifth place as the leading midfield runner.
Magnussen, whose seat in the winning Ganassi Cadillac for Saturday's Sebring 12 Hours was taken by Neel Jani, was on course for seventh on his return before the late retirement of both Red Bulls elevated him to fifth - the team's best result since the 2018 Austrian Grand Prix.
Speaking after the race, Magnussen said: "That was a lot of fun, I enjoyed it a lot. It's so good to be back in this position, I've just got to say a massive well done to the team.
"I mean, we were the strongest car in the midfield, I could actually see the Mercedes almost the whole race. I know we got a safety car there at the end, so that helped a little on that, but it was just a different story to in the past.
"I keep saying this all weekend that I just can't believe this opportunity that I've got here. P5 today, crazy."
Magnussen acknowledged that his neck was feeling "pretty stiff" but said he was "better than I thought I would be" from a fitness perspective.
"I was worried I would be way more tired," he said.
"But sometimes when you're in a good position, you get some extra energy and I certainly - I was fine."
Magnussen said however that the team is not getting carried away by its showing in Bahrain, and said it will remain focused on the midfield battle in the races to come.
"The midfield is our focus and we know we got lucky today with the Red Bulls," he said.
"If we can finish P7 in Jeddah, then it's the same as today basically. We were just lucky today getting four more points than we would have with a P7."
Magnussen explained that he "used the tyres too hard" in his opening stint which caused him to pit two laps earlier than planned, but managed to extend his second stint - also on soft tyres - by the same amount to get back onto the team's original strategy.
"So [we] managed that really well with the engineers, getting the right amount of pushing in the critical corners and all that," he said.
"I'm certainly getting more into it, but I think everyone is now with these cars, just learning more, getting more on top of it.
"It's always like, if you put a shoe on that fits you, then when you start walking it still gets better, so it's a bit like that."
