Pierre Gasly and Max Verstappen will be a strong pairing but they could pose "problems" to Red Bull, according to Toro Rosso boss Franz Tost.
Gasly, who scored 29 of the Italian team's 33 points in 2018, will be joining the Dutchman at the senior team having been selected to replace the Renault-bound Daniel Ricciardo.
While his arrival at the front of the grid isn't being met with the same expectation as Charles Leclerc at Ferrari, Tost does think the 22-year-old is up to the task.
"Pierre is really doing a good job," he said. "He improved a lot as the season went on and I am really happy with him."
As for what Red Bull can expect next season between Verstappen, who has established himself as the lead driver, and Gasly, the Austrian thinks fans could be in for a surprise.
"I expect it will be a good partnership," he stated. "Both drivers want to be successful and there will also be earlier or later problems over there [at Red Bull] during a race.
"But that is normal in F1, drivers are fighting for positions and both drivers want to have success. But I expect, as both drivers are professional, they will sort it out in the best possible way."
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As for Toro Rosso, they will have a new line-up in Daniil Kvyat and Alexander Albon and a role to play in making Red Bull's switch to Honda engines for 2019 as successful as possible.
“If we can help Red Bull win the title by being the guinea pig in terms of developments, then that’ll happen," Tost claimed.
“We’re happier with this role because we’ll see the fruits of our labour as well. Not only in terms of the engine, but we’ll have advantages in other aspects as well because of the partnership between Honda and Red Bull.
“The full backside of the RB15 will come our way as well.”
