Both Magnussen and Grosjean are leaving the sport after being replaced by rookies Mick Schumacher and Nikita Mazepin for the 2021 F1 season.
Magnussen and Grosjean spent four years as each other's teammates at Haas, eventually building a healthy relationship tested by several on-track incidents.
In Abu Dhabi Magnussen paid tribute to his now former colleague, saying Grosjean's "phenomenal" one-lap pace made the Dane a better qualifier.
"I think what I experienced with Romain is an incredibly fast driver on one lap, not that he's not quick in the race, he's super quick in the race as well, but especially in one lap, he is something special," Magnussen said.
"I think having him as a benchmark, especially in qualifying, has really made me a better qualifier. You seriously need to get it right to beat Romain in qualifying.
"It's hard to imagine that anyone can be much quicker than him, because on his day, he is phenomenal on one lap."
Haas unable to give Grosjean farewell Formula 1 test Mercedes willing to give Grosjean farewell F1 test Grosjean “wouldn't race any car without a halo” after crash Grosjean gives full account of Bahrain fire escape
Magnussen believes Grosjean is underrated after facing criticism is recent years for being error prone.
"I think F1, it's funny, because Romain has made some mistakes in the last few years that have gained a lot of attention, and I think taken away from him some of the credit he should have been given for his abilities in the car," Magnussen added.
"I think my personal feeling is that there's not a lot of people that can be faster than him on one lap."
After six full-time F1 campaigns, Magnussen will move into sports cars with Chip Ganassi Racing, driving the American team's Cadillac Daytona prototype in the IMSA Weathertech Sportscar Championship.
Grosjean is recovering from the burns on his hands, which he sustained during his fiery Bahrain Grand Prix crash and is taking his time to heal and consider his future options in the sport.
Before his accident the 34-year-old was mulling over a potential IndyCar move, but since Bahrain he has admitted he might have to reconsider his plans.