Premier League legend Alan Shearer has sent a clear-cut warning to Manchester United fans after the Red Devils’ 4-3 FA Cup win over Liverpool. Manchester United produced a sensational performance against Liverpool at Old Trafford on Sunday and punched their ticket to the FA Cup semi-finals. Amad Diallo, who is suspended after his sending-off, broke the deadlock in extra time and sent Liverpool crashing out of the FA Cup with his superb match-winner. Following the thrilling 4-3 quarter-final contest at Old Trafford, United learned that their semi-final opponents would be Coventry City at Wemble…