The former Tottenham striker returned to Los Angeles late last week after representing the Republic of Ireland in two Euro2012 qualifiers in five days and Arena decided to start him on the bench for the meeting with the Rapids on Friday.
But the Galaxy boss was happy with the performance of Keane who will jet off in October for his country's final two qualifiers as they chase a spot in next year's European Championships in Poland and Ukraine.
"Well Robbie [Keane] is a real warrior. I think he's not happy with me that he only played 30 minutes. If Robbie had his way he would probably have played 90 minutes," Arena said.
"We've got to think about the player. We didn't want to put him in a position where he would get hurt and 30 minutes is difficult, maybe 45 minutes is a little bit better so he gets a chance to warm-up a little bit better and have a little more time to get into the game but I thought our guys played so well in the first half that I wanted to reward them and keep some of them in the game a little bit longer.
"But Robbie is going to be a real plus for us this year and we're looking to get him in on a consistent basis since he's only practiced with us a couple of times. We will have him for the next four weeks or so before he goes."