Keane was playing with Ireland's national team Tuesday night in Dublin, where it defeated Kazakhstan, 3-1, in a World Cup qualifying match.
The forward then flew nearly 14 hours, or 5,157 miles, to be at StubHub Center on Wednesday afternoon in time for the Galaxy's game that night against the Montreal Impact.
It was a key game for the Galaxy, which is in a scramble to make the playoffs in hopes of winning its third consecutive Major League Soccer championship.
Keane "walked in and said he's ready to go and he'd give us as many minutes as we need," Arena said.
Keane did not start the game. But with the Galaxy already without Landon Donovan, its other star forward, the team struggled to score and was locked in a 0-0 tie with Montreal at the 60th minute.
Arena then sent Keane, 33, into the game. Eight minutes later, the Galaxy's Kofi Opare kicked in the first goal of his MLS career, giving the Galaxy a 1-0 victory.
"We needed to get Robbie on the field and his presence was spectacular," Arena said. "He gave the team confidence, energy, the technical ability that we needed to try to break this team down."
Keane said that "at 60 minutes there was not much happening and [Arena] put me in."
"This was a massive game for us," Keane said. "I think it was very, very important for us to get the three points" with the win. "That was one of the reasons I wanted to come back because I knew this could put us into the playoffs."