I don't think that really has anything to do with it.
One is ready for the NHL and the other isn't.
Their linemates, ice time, and usage are fairly similar.
Galchenyuk is picking up where he left off in 2010-11 (not proving that it wasn't a fluke and all Yakupov, that argument was garbage as really the only time they saw the ice together was the PP), whereas Grigorenko is starting to make me worry that his 2011-12 first half look like an outlier rather than a change for the better.
People won't like to hear it, but he looked like a different player in his first half last season.He not only started to recognize that he had to play in his own zone, but also cut out some of the crap in his game, started skating more, and while he will never be a physical player, he started using his use size to create an advantage for himself.
Post-injury/mono, he's kept up his defensive play (although he still has warts at the NHL level), but the dipsy doodling came back (not as much in the NHL yet, was a big issue in the Q, remains to be seen if it becomes an issue as he gains confidence), he's not engaging at the same level as he was last year, and he has really only one gear again, which is slow.
He can still probably make things work if he's going to be slow, but he's going to have to improve the other facets of his game.
My hope is that he's still feeling the effects of mono (it can take a few months for normal individuals to recover but I've heard some say it is especially hard to recover from for athletes and can sometimes take almost a year) and that his apparent improvements weren't an outlier, but I have been worried about it since the beginning of this season.