Mari is set to join Udinese on loan for the remainder of the season, while Kolasinac has signed with Marseille after having his contract terminated.
Asked why both players have left the club, Arteta said: "Well for different reasons, with Kolasinac as well the gametime that he's had has made it really difficult for him to be in the team.
"His contract was ending at the end of the summer. so we believed that it was the right option to not execute that possibility.
"He wanted to go, he has a good option, and it was a good option for the club as well. With Pablo it was much trickier, with the injuries and the history he's had with the club since he joined.
"He's a phenomenal professional, and he hasn't had the minutes this season - it was going to be difficult for him to get the minutes - and a good opportunity came, and we have to be open to that."
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