Jose Mourinho has insisted Frank de Boer's "incredible" sacking by Crystal Palace should not come as a surprise.The Dutchman, 47, was dismissed by Palace earlier on Monday after losing his fourth of four Premier League fixtures at the club in Sunday's 1-0 defeat at Burnley.His departure comes only 77 days after being appointed to his first job back in management since last season being sacked by Inter Milan in 85, but despite serving so little of his three-year contract, Mourinho believes it has become near-routine."I was sacked (by Chelsea) as a champion, (former Leicester manager Claudio) Ranieri was sacked as a champion," said the Portuguese, also once of Inter."Frank de Boer is sacked after four matches and next season something incredible again, so nothing surprises me. That's the world where we are. "That's the world where many of you enjoy and also contribute to the kind of pressure that managers are involved."But in football nothing surprises me. Nothing surprises me."Joey Barton most recently played in the Premier League for Burnley, a club that has thrived under the stability provided by manager Sean Dyche, who in the summer was interested in the Palace vacancy before De Boer's appointment.He also played for both Newcastle and QPR, two of English football's more volatile clubs, and wrote on social media: "I feel sorry for Frank de Boer. I mean what is a guy to do with four league games? Premier League is nuts."Upon his appointment at Palace, De Boer said he had been offered the Liverpool job after Kenny Dalglish's departure in 2012, while Jamie Carragher remained a regular, and the retired defender wrote: "Was it the players or his style that failed him? Best man to give his opinion: Big Sam (Allardyce)."De Boer also played for Rangers while John Hartson was at Celtic, and the one-time striker added: "Frank de Boer (will be) naturally disappointed. But the positives are he walks away with millions. Pay off. And he's back in work in six months."