Jenson Button would "strongly recommend" Kevin Magnussen to any team with a seat still available for the 2016 season as he is "talented and fast."
Dropped from a race-seat to a test driver role after just one season racing for McLaren, Magnussen is determined to return to racing in 2016.
The Dane, though, is running out of options.
Lotus, soon to be Renault, and newcomers Haas each have a seat available while Manor, who will be powered by Mercedes in 2016, have yet to confirm their line-up.
Button suggests they all look to Magnussen, who he feels is the "best" driver still on the market.
"I do not know if he has the chance to take one of the few free seats in Formula 1, but he is clearly the best driver available on the market," Button told Danish publication BT.
"I would strongly recommend him to any team that still has a place."
He added: "He is good enough to help a team to progress. We were team-mates last year and I I know what type of driver he is; he is talented and fast.
"In F1 where you have to continually adapt to new situations and be able to learn, he learned a lot in the 12 months we have worked side by side. Kevin at the beginning of 2014 was very different at the year-end."