Take a look at both Arenas's and Lewis's stats before and after the trade.
Lewis's numbers are better across the board. He's not just playing more minutes and taking advantage of that, but he's also shooting a higher percentage from the field. Arenas, meanwhile, has struggled mightily and is playing the fewest minutes per game of his career. He's also making just 36.4 percent of his shots, which obviously isn't very good at all.
Remember, of course, that this is only looking at the current season and just a small sample of games, so Lewis's numbers could drop off and Arenas could start playing better as he becomes more comfortable with his role on the Magic. But right now, even if the trade wasn't about Lewis replacing Arenas on the roster and making the Wizards better right now, Lewis is playing better overall and is doing a decent job filling in.