Keane (below) was given permission to miss Ireland's friendly with Greece this week, with Galaxy not keen on such a key player travelling at a crucial point in their campaign.
Ireland's record goalscorer opened the scoring against Seattle before tapping home the third to put an emphatic look on the scoreline.
"He's been simply outstanding since his return from the European Championship," said Arena, after Keane moved onto the 19 mark for the campaign.
"One could argue he's been the best player in the league, unbelievably important to our team. He's been healthy and in just about every game since he's returned. He's simply been outstanding. What can I say? He's been great."
Keane defended his decision to poke the ball over the line for his second – Christian Wilhelmsson's strike was bound for goal before his touch.
"My coaches when I was a kid always said just make sure the ball hits the back of the net," he said.
"I wasn't too sure who was around me so I was just making sure, because they cleared a few off the line tonight so I wasn't taking any chances. It doesn't really matter who scored as long as we win the game."
He warned his team-mates not to be complacent after making a giant step towards retaining their title.
"We got a good win tonight, but the job's only half done. So if the lads think for one second now this is over, they may think again, because as we know in football it's a funny old game and anything can happen.
"We'll enjoy tonight, enjoy the win, but as soon as we're back on the training pitch, the focus is on next Sunday."