The 31-year-old midfielder moved to the Gunners on loan from Spartak Moscow and a deal went through despite a medical showing up his back problem.
The Sweden international was an unused substitute in the 6-0 mauling at the hands of Chelsea, but he could be in line to make his long-awaited debut against Swansea this evening (7:45pm).
And he has admitted he was surprised Arsenal swooped in for him.
"It was bewildering (joining Arsenal) to say the least," Kallstrom told Swedish publication Sport Expressen. "It went very quickly.
"Then everyone at Arsenal took care of me very well, it's been great care and rehabilitation.
"Everything has been well managed and I have received good care and rehabilitation. It has gone very well.
"It was surprising. It was nothing I had expected. Of course I was surprised.
"I've had a good career, they needed an experienced player and I fit the profile and took the opportunity."