Timber featured in victory over Manchester United in LA, having missed almost the entire last season due to knee injury.
Arteta said: "He’s like a new signing for us, a big addition to the team.
"Now he’s getting the rhythm because obviously he’s missed a lot of football in the last year or so. Just look at him… his pressing, his leadership, his composure."
On playing Timber at centre-half, Arteta added: "Making one change and not two. It's simple and as well there is so much chemistry in that right unit.
"That relationship has been there for a long time and I believe at the moment that it's better just to touch one thing. Now he's getting the rhythm because obviously he's missed a lot of football in the last year or so.
"We asked him to play in a different position than we had in mind due to the numbers and the players we have at the moment but I'm really happy."