Only reason why Rasmus' endurance failed in the 2nd half was because he overworked himself in the cage. The coaching staff and Rasmus acknowledged this and they have corrected it
http://www.theglobeandmail.com/sports/baseball/jays-outfielder-rasmus-happy-to-have-dickey-johnson-on-his-side/article8773782/
"Consistency at the plate is the first order of business in 2013.
Last campaign, Rasmus hit .291 in June but batted just .191 in July, .167 in August and .209 in September.
“This year I’m looking to pace myself a little more,” he explained. “Sometimes I get a little carried away with trying to do too much or when I start struggling, get in the cage and taking more hacks and working on things a little more than I should.
“I’m going to try to take it easy and try to be more consistent throughout instead of just busting it hard at the beginning and kind of falling it off.”
Rasmus also hopes not to let things fester mentally.
“This is a competitive game and I don’t like to lose,” he said. “Sometimes I take that to the extent of overworking myself instead of just knowing that I’m good, get out there and just play the game.”