From third and fourth on the grid, Verstappen was beaten into the first corner by Russell, who then defended robustly through Turns 2 and 3 at the Baku Street Circuit.
Although Verstappen eventually managed to reclaim third from Russell, the damage caused prevented the reigning F1 champion from making any inroads into the front two – eventual winner in team-mate Sergio Perez, and runner-up Charles Leclerc in his Ferrari.
In parc fermé following the race, Verstappen then confronted Russell, using the f-word on a number of occasions, according to the British driver, and calling him a “d***head” as he walked away.
Asked by Speedcafe about the incident, Horner said: “You've got two competitive guys.
“Max was obviously aggrieved because it (the impact) did a significant amount of damage to his car, which then compromised his race.
“So, of course, when they take their crash helmets off, you know, Max has never been shy to show his emotions.
“He was annoyed that…he's not afraid of hard racing.
“But I think the damage caused to his car, that compromised his race, which was what he felt aggrieved by.”
Verstappen “a hard racer”
Pointed out to Horner that Verstappen would have defended just as vigorously against Russell if the roles had been reversed, he added: “We know Max is a hard racer, always has been and always will be.
“The fact they made the contact, that's the annoying bit, and particularly for Max where it's then compromised his race.
“The damage was done at turn two where George has locked up and gone into him. Sometimes that's racing.
“It was quickly turn the page, he voiced his opinion and quickly focused on the next event.”
Verstappen had to settle for second in the grand prix behind Perez who completed a stellar weekend to add the main race win to his sprint triumph and close the gap on his team-mate at the top of the drivers' standings to six points.