The £50million Blues striker has never been able to reach his best in a Chelsea shirt but Arteta is appalled by some of the criticism which has come his way.
"I don't like the way he has been treated because I have known him since we were 16 years old and I really appreciate him," Arteta told the London Evening Standard.
"He is a friend of mine. Football is like that - it doesn't matter what you have done in the past, it is only what you have done in the last few weeks and he has been criticised.
"Mentally he is really strong, even if someone doubts him, because he has been dealing with pressure since he was 17. At Atletico, he was captain by that age."