Christian Horner believed that Mercedes had made "strong" steps with their recent upgrades, but expressed confidence in Red Bull warding off any threat over the rest of the Formula 1 season.
Max Verstappen delivered one of Red Bull's most comprehensive victories of the season so far at the Spanish Grand Prix to extend his Drivers' Championship lead to 53 points.
The Mercedes team, who took second and third place with Lewis Hamilton and George Russell, had their first race at a non-street circuit with their upgraded car.
The Brackley team moved up into second in the Constructors' Championship, and showed encouraging race pace to prevent the second Red Bull of Sergio Perez from recovering to the podium positions.
When asked by Sky Sports F1 if he had any concerns about Mercedes' improved showing at the Spanish Grand Prix, Red Bull Team Principal Horner responded: "They were still 23 seconds behind at the end of the race.
"They've made a good step, but we've got some stuff in the pipeline. They've had a big upgrade, we've got some bits coming later into the season.
"But for sure it's a big step. They are a quality team, and they're going to be putting us under pressure in the second half of the year.
"The form is moving around. You look at Lando Norris' lap in qualifying, the Mercedes looked strong in the race too.
"The Ferraris didn't have the pace that we thought that they might, so the form behind us does seem to be moving around quite a lot at the moment. So it's difficult to predict who's going to be where at which event."