The Gunners’ boss has recently switched from a 4-3-3 formation to a 3-4-3, and to good effect.
Arsenal were unbeaten using the new system going into Sunday’s north London derby at Spurs, and looked good value for a result.
But Keown criticised the manner of Arsenal’s defeat having controlled much of the possession but conceding two goals to “sloppiness”.
“Arteta needs to decide which way he wants his team to play — three or four at the back — and buy players who suit his preferred system,” Keown told The Mail.
“Harry Kane and Son [Heung-min posed problems. We saw Shkodran Mustafi diving in — maybe that’s because he doesn’t feel he’s going to get cover from [David] Luiz behind him.
“Choose your system, Arteta, and buy accordingly.”
Arsenal had gone in front thanks to a thumping opener from Alexandre Lacazette after just 16 minutes.
But their lead was short-lived as a poor pass from Sead Kolasinac allowed Son to steal the ball ahead of Luiz and level the scoring.
Despite all Arsenal’s dominance of the ball they were then undone by a set piece late on as Toby Alderweireld headed home the winner.
Keown lamented how the sloppiness was again Arsenal’s undoing, and claimed “both goals could have been avoided”.
Speaking after the match, Arteta suggested his teams inability to hold onto the lead could be attributed to “fatigue”, but conceded the mistakes cost his side.
“We have some moments when it wasn’t about that [fatigue], it was about attention, individual errors or sometimes the opponent pushes you in the final 10 minutes.
“There have been various things that have cost us games. Sometimes errors, sometimes game management. The margin is a lot, at the moment we have to improve.
He added: “I am really proud of our players, to play the way we played. I have never seen them before dominating the opponent and nullify them as much as I’ve seen today.
“But we gave them a goal again and we conceded a set-piece and, at this level, you cannot do it.”