Key joined the team from Sauber in 2012, and has overseen steady improvement ever since, with the Italian squad rising from ninth in the constructors' championship that year to seventh in 2015. This year has been particularly impressive, with Toro Rosso landing several notable results to sit sixth in the standings at the summer break, despite having to also contend with a late switch from Renault to 2015-spec Ferrari power units.
Tost says such progress proves Key has all the qualities to join Red Bull's Adrian Newey as one of the most celebrated designers in the sport.
"James has developed a lot in the last few years, and he has built up an excellent team around him," Tost said. "Adrian Newey is simply much more experienced, but I am convinced that James will be one of the top technical personalities in F1 in a couple of years."
Asked whether Key can succeed Newey as F1's foremost designer, Tost added: "Why not? But it will depend much on where the regulations go. As always in life it is a matter of being in the right place at the right time."
Challenged as to whether there was any concern that Key could be courted by rival teams, Tost - speaking before James Allison's departure from Ferrari - stressed: "Of course it is always a person's ambition to do as they want - but nevertheless James has a contract with Toro Rosso and I expect that he will respect this contract.
"This is the point where I have to use the confidential word! (laughs) And we want to keep it [the length of the contract] confidential."