The Lakers' problem with Brian Cook was, if Radmanovic is healthy, Cook is entirely redundant. It was a head-scratcher when they signed Radman in the first place, and an even bigger head-scratcher when they extended Cook with Radman already on board.
Evans is a guy who does a little bit of everything, but not a lot of anything. He's the basketball version of the 'replacement-level player'.
Thanks to Tony Battie's injury, the Magic found a game plan they believe will work, playing Rashard Lewis at PF and surrounding Dwight Howard with four quality 3pt shooters. Their problem was, they were short a PF and didn't have enough shooters on their bench--Redick sucks and Pat Garrity is done--and instead of having backup centers who can shoot from mid-range, they now have Adonal Foyle, so Ariza was struggling to find space for his 'offense', which consists of Ceballos-style garbage buckets and occasional drives to the basket.
(Ariza looked good with the Knicks, but most rookies look good there, because the veterans underachieve. He came to the NBA as a 6'9" athlete with few skills, and he's pretty much the same player today.)
Obviously, the Lakers would like it if Ariza suddenly blossomed two years from now, a la Gerald Wallace; however, Wallace spent two years carrying more than his share of the load in a historically bad offense before he started to look like he does today. More likely, Ariza will come home, get comfortable, his improvement will tail off, and he'll have a short career.
IMO this is the order of importance of the Lakers' priorities:
1. Avoid the luxury tax by clearing salary
2. Dump Brian Cook's contract
3. Send Cook as far away as possible
...for all of Cook's weaknesses, top-level shooters with his size are unusual, and in Houston or Phoenix, he could cause a lot of trouble coming off the bench. And since the Lakers are going to get waxed in the first round anyway, I'm sure they'd prefer to avoid any more criticism of their front office...
4. Diversify their bench a bit by replacing Evans with Ariza.
So, even if Ariza is a complete bust, the Lakers have already accomplished most of what they set out to do.