The Spanish striker joined the Blues from Liverpool for £50million in January 2011 but has struggled during his time at Stamford Bridge.
Artera and Torres began playing together in their teens and the Gunners man feels the Chelsea attacker has been harshly judged ahead of tomorrow's meeting between the two clubs at the Emirates Stadium.
"I don't like the way he has been treated because I have known him since we were 16, 17 and I really appreciate him," he told Arsenal's official website.
"He is a friend of mine. Obviously, 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.
"He has been criticised. Mentally he is a really strong guy even if someone doubts him.
"When you play as a striker and they pay £50 million for you, you are under huge pressure.
"When the ball isn't going in the net, it is even more difficult because people always look at that. For a striker, sometimes it is not fair. For a midfielder or a defender it is a bit easier to adapt."