Haas have yet to decide who will complete their F1 driver line-up for next season.
Magnussen has a multi-year deal with the team, while the second seat is between Mick Schumacher and Nico Hulkenberg.
Despite impressing on his return at the season-opening Bahrain Grand Prix, Magnussen’s form has dipped in the second half of the season.
Since the Austrian Grand Prix, Schumacher has often been the quicker of the two Haas drivers, particularly on a Sunday.
Speaking to Sky Germany, Ralf said that Magnussen’s seat should also be under threat for next season.
“Anything is possible, of course, especially if we look at the last few races,” he said.
“Who knows, Magnussen may not be so firmly in the seat anymore. If we look at the races since Silverstone, we have to conclude that Mick is the more stable and better of the two drivers – and who has also had some bad luck with wrong strategies.
“In the last race [Mexcio], Mick was sometimes up to six-tenths faster than Magnussen – so let’s wait and see for the next few races. I hope Haas make a decision soon.
“Maybe, next year we will have two Germans on the Formula 1 grid.”
Recently, team boss Guenther Steiner explained that it’s “still 50-50” on who will drive for the team alongside Magnussen in 2023.
"I'm honest: It's still 50-50,” he explained. “Nothing has been decided yet," he said. “We will logically not make a decision between Austin and Mexico simply because time is not there.
“The next time would be the week after Mexico, because we don't have a race then and have more time to deal with it."