Red Bull Formula 1 chief Christian Horner says Max Verstappen acts as "an enormous barometer" to go up against as a teammate amid Pierre Gasly's struggles at the team through the first half of the 2019 season.

Verstappen has enjoyed one of the best runs of form in his F1 career so far this season, winning two of the last four races before the summer break and taking a second-place finish in Hungary to put himself just seven points off Mercedes' Valtteri Bottas for P2 in the drivers' championship.

By contrast, Gasly has only scored a best finish of P4 and picked up 63 points to his teammate's 181 through the opening 12 races of the year, leading to speculation about his future with Red Bull.

Speaking over the Hungarian Grand Prix race weekend, Horner sympathised with the task Gasly faces going head-to-head with Verstappen at Red Bull amid the Dutchman's excellent run of form.

"I think it's very tough. If you go up against Verstappen at the moment - for me arguably he is the most in-form driver on the grid - that’s an enormous barometer for any driver to go up and be measured against," said Horner.

"As a competitor, you always want to measure yourself against the best that you can. So, he’s a benchmark that obviously Pierre is measuring himself against."

Horner has already stressed that Red Bull is not considering switching Gasly out of the team mid-season, but said it "desperately" needs him to start picking up more points soon if it is to contend with Ferrari for second place in the constructors' championship.

Verstappen has scored the lion's share of Red Bull's points this year, continuing his streak of top-five finishes that dates all the way back to last year's Belgian Grand Prix.

"I think Max, not just in the last three races but during the course of the whole of this season, indeed pretty much Montreal last year, he’s been driving incredibly well at a very high standard," Horner said prior to Verstappen's second-place finish in Hungary.

"It’s certainly started to come together over the last few races. The win in Austria was a watershed moment. We were competitive at Silverstone, and obviously the race, the excitement of [Germany] was a phenomenal performance by him, to keep his head, to have the pace in all varying conditions, and come out on top and win the grand prix.

"He’s in great form and hopefully we can continue that form into the summer break and out of the other side as well. He’s certainly delivering at an extremely high level."