Pierre Gasly will replace Daniil Kvyat at Toro Rosso from this weekend's Malaysia GP for an unspecified number of races.
Gasly is expected to land the race seat to be vacated by Renault-bound Carlos Sainz at the end of the season and Toro Rosso have decided to use the upcoming races, starting at Sepang, to evaluate Gasly in a race weekend.
Although reports have suggested Gasly will be in the car for the Malaysia and Japan double-header, the team did not confirm how many races Kvyat would be sidelined for.
"This is not a case of goodbye for our Daniil, as he still remains part of the Red Bull family," read a Toro Rosso statement.
The Russian has struggled for points since losing his Red Bull seat to Max Verstappen in May 2016 and claimed just four in 14 races this season. Team-mate Sainz has contributed over 90 per cent of the team's 2017 total.
"Scuderia Toro Rosso was established by Red Bull to bring youngsters from its Junior Programme into Formula 1 and that's what we are doing by giving Pierre this chance," said team boss Franz Tost.
"He is the next in line at Red Bull for this opportunity and he has shown that he deserves it, having taken the 2016 GP2 title and this year being very competitive in the Super Formula series in Japan. He really has a valid chance of winning the title, as he is only half a point behind the leader.
"The driver switch gives us an opportunity to make a more informed decision regarding our 2018 driver choices. For a variety of reasons, some of them due to technical problems, but others being mistakes of his own making, Daniil Kvyat has not really shown his true potential so far this year, which is why we are standing him down for the next races. This will give us the opportunity to evaluate Pierre on track during a proper race weekend."
Gasly's title-contending position in the Japanese Super Formula series means he is likely to return to the championship for its season finale at Suzuka on October 22 - a date which clashes with F1's US GP in Austin.
Kvyat therefore appears likely to return in Texas, although Gasly could yet feature again from Mexico onwards.
Kvyat crashed out on his last appearance in the rain-hit Singapore GP, a race in which Sainz finished a career-best fourth. The Russian last scored points at the Spanish GP in May.