The Republic of Ireland international continued his stellar recent form with a match-winning double in the 3-0 Western Conference final first leg on the weekend to take his season goals tally to 20 and playoff record to four in his last two outings.
Arena was complimentary of Keane following his brace in the midweek win over San Jose Earthquakes and he was again full of praise for the former Tottenham star in the wake of his more recent exploits.
"I've said it a number of times: I think he's been simply outstanding since his return from the European Championships," said Arena.
"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 I would think since he's returned he's simply been outstanding. What can I say? He's been great."