#4 Kevin Magnussen
It's never easy going up against Jenson Button in the same car, just ask the aforementioned Perez. Kevin Magnussen has established himself as a solid driver in Formula 1, but his route to Haas was far from a straightforward one.
The Great Dane had a solid debut campaign in 2014 but was dropped in favour of Alonso for the following year, it's never easy going up against two world champions. 2016 saw him become a part of the revived Renault outfit, and K-Mag significantly out-performed Jolyon Palmer (albeit not the best of drivers).
2018 was arguably his best year, though, as Magnussen placed ninth in the driver's standings, and could've been higher if Haas didn't make some rookie errors of their own (such as in Australia).
Yes, his defending is too robust at times, and that will land him in even more trouble if he doesn't amend this, but Rome wasn't built in a day and at just 26-years-old, Magnussen has plenty of time to improve.
In short, Haas have a very solid driver line-up, if the VF-19 is fast, the pair of Magnussen and Grosjean will deliver.