Christian Horner admits that a possible reliability concern for Max Verstappen during the Saudi Arabian Grand Prix made his "heart immediately sink". Verstappen had experienced a driveshaft issue in Saturday's qualifying session which left him unable to progress beyond Q2. The Dutchman consequently started from 15th on the grid but eventually found himself running in second behind teammate Sergio Perez. While another 1-2 result looked to be on the cards for the squad, Horner was left fearing the worst when Verstappen reported a potential problem with his RB19. Horner on 'biggest point of concern' in race "I think once we…