Red Bull have carried out a precautionary gearbox change on Max Verstappen's car for the remainder of the Miami Grand Prix weekend.
After a brush with the wall in FP1, Red Bull opted to swap out the Dutchman's gearbox for FP2, meaning he spent the first part of the session stuck in the garage.
As the change is within the limit of four for the season, Verstappen will not face a grid penalty.
"There was no damage we could see, but you don't want to take the risk and then be sat on the side for the whole session," Horner told Sky Sports F1 during FP2.
"We decided it was better to sacrifice a bit of time and we got lucky with the red flag (for Carlos Sainz's crash)."
Gearbox changed, only for more issues follow
But shortly after he left the pits at the halfway mark of FP2, Verstappen slowed with more issues – his rear-right brake appearing to catch fire.
"I can't do anything, I'm really sorry," Verstappen reported over the radio as he crawled back to the pits, unable to steer his car properly.
Lance Stroll was almost caught out by the slow-moving Red Bull and had to dart off the track to avoid making contact, leading to a sweary radio message from the Aston Martin driver.
The incident was noted by the stewards, who decided to take no further action.
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Horner reflects on "horrible" day for Verstappen
Horner subsequently confirmed that Verstappen's latest issues were due to a hydraulics fault.
Verstappen ended first practice in third position, but was unable to get a lap on the board in the second session.
"He's had a horrible day," Horner summed up after the first two hours of practice.
"He's driven out of the pit lane and had a hydraulic issue, which has unfortunately caused some damage to the rear brakes.
"He got a few laps in. I think he's had five timed laps today, which were all competitive.
"Thankfully, with the red flags, he's not lost too much, but it's frustrating not to have that track time."
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