Juan Rivera comes from one of the most pitcher friendly parks in Baseball, and has consistently hit substantially better on the road.
On average the Angels and there opponents only score 86.4% of the runs they score on the road and 82.5% as many home runs and so one would expect hitters to have somewhat lower production from Angels players. He is moving to a hitter friendly park in Toronto, where teams score 105.8% of the runs that Toronto and their opponents score on the road, and where they hit 135.8% of the home runs.
( http://espn.go.com/mlb/stats/parkfactor )
Juan Rivera has rough home stats.
2009 Road BA-.308 OPS -.851 Home BA- .270 OPS-.762
2010 Road BA-.283 OPA-.819 Home BA- .223 OPS-..625
If we average his road OPS over the past two years we get .835 which would be good enough for 14th among all outfielders in baseball last year.
This is not even accounting for the fact that he is not moving to a major league average park, but a park that should give him a significant boost in terms of both OPS and especially home runs.
He is moving to a hitter friendly park in Toronto. He is coming off a poor season in a while, and so his trading value is fairly low.
Even if he isn't in the Jays long term plans, I think he is likely to have more value towards the trading deadline and is worth holding on to till later in the season.