Guus Hiddink has 'no doubt' Per Mertesacker's red card was warranted

Chelsea interim manager Guus Hiddink says he has no doubts regarding the legitimacy of the red card issued to Arsenal defender Per Mertesacker, after the German lunged in with a last-man slide tackle on an onrushing Diego Costa.
Holding a one-man advantage, the Blues went on to win the derby match 1-0, with Costa scoring the only goal of the match. Afterward, Hiddink affirmed his belief that there was indeed contact in the foul call that reduced Arsenal to 10 men.
"There was no doubt. You could see it was a beautiful through pass from Willian to Diego, he could have run straight through to Petr Cech but he was tripped," Hiddink told the Blues' official website. "I had some doubts about the Cesc Fabregas penalty claim, and I can understand why the referee didn’t give it, fair enough, but Mertesacker’s red card was obvious."
Costa has come under fire throughout much of the year as his form and his temperament makes him a figure that many opposition fans love to vilify. But, under Hiddink, the Spanish striker is scoring once more and expressing himself onto matches, too.
The change comes down to a shift in focus, according to Hiddink.
"He (Costa) is focusing on his first job," Hiddink told reporters. "There are no big conversations (about temperament) but we talk. (I tell him) 'Don't waste any energy which you don't need. You need the energy to put into what you're good at.' He's focusing on that and finally it's paying off.
Costa pushed and shoved and ran and prodded his way through the Arsenal back four for about 70 minutes before being substituted with what looked like a knee injury, something Hiddink confirmed after the match.
"He got a knock on his knee," he said. "I don't know if it's slight or more severe but we will check on it."