All players do it. When Stevie G had the league in his grasp, he fell over. When John terry needed to kick a ball 12 yards to win the champions league, he failed. Where was Ronaldo hiding last weekend in the CL final? Where was messi in the World Cup final in 2014?
Players have bad games, even the very best. trying to rewrite history to say he's not that good makes us look like Southampton fans. He's a top class striker, and the fact you think one of the worlds richest clubs will buy him tells me you know that. So just accept that he's a good player who's likely to get better, and he's given us 3 good years with no sign of the team getting better. I'd leave if I was him, so I won't hold it against him if he does.