• Chelsea will struggle to finish in top four, says United manager• David de Gea set to return for Stamford Bridge showdownSir Alex Ferguson is used to new challenges. In Manchester United's two decades at the top he has experienced intense title rivalry with Arsenal and then Chelsea, though now it appears Manchester City and Tottenham Hotspur are the greatest dangers. And, though United visit Stamford Bridge on Sunday afternoon, a ground where they have not won in the league for a decade, Ferguson believes Spurs a month later could prove the tougher trip."We'll certainly be glad to get the two games out of the way," the United manager says. ...