Ricciardo's progress was halted by the hand injury that he received at his third appearance in Zandvoort, which cost him five races.
However, in Mexico he stunned the F1 paddock by qualifying fourth and hanging on to a valuable seventh place in the race.
Tost, who steps back into an advisory role at the end of the season, says he knew that Ricciardo – who drove for the team in its Toro Rosso guise in 2012 and 2013 – would get up to speed.
"Absolutely no doubts," he told Autosport. "I didn't have any doubts, because he's experienced, and he couldn't have lost the ability to drive an F1 car fast.
"Once you know this, you don't lose it. And what he missed maybe in the past was the trust in the car, trust in the team. Therefore, he couldn't perform on the level which we know he is normally able to do."
Tost conceded that even though Ricciardo looked quick from the start of the Mexico City GP weekend the Australian had exceeded expectations in qualifying.
"It was a positive surprise," he said. "I expected him to be within the first six. But to be in the fourth position was really good.
"A very good performance in qualifying as well as in the race, especially at the beginning, when he could keep Lewis Hamilton behind him.
Source: Autosport