Red Bull team principal Christian Horner expressed "concern" for Max Verstappen in the wake of Sergio Perez's early retirement from the Canadian Grand Prix.
From 13th on the grid at the start following a crash in the second qualifying session on Saturday, Perez had made his way up to 11th when he suddenly slowed on lap seven before pulling over into a run-off area at the Circuit Gilles Villeneuve.
Red Bull fear gearbox issue for Perez
"He's lost drive and we think it's a gearbox issue," said Horner on Sky Sports F1. "Obviously, we need the car back.
"Really disappointing for Checo. I think he could have been very quick out there today."
The retirement represents the fourth this season for the team due to a mechanical issue.
Asked whether he was concerned about a similar issue for Verstappen occurring later in the race, Horner replied: "Of course, it's always a concern.
"We've just got to get our heads down now and do the best we can with Max."