Plekanec and Eller too.
MB and MT were condemned to make the Playoffs in that shortened season. Not to groom a raw rookie out of OHL. Galchy did very well on LW with Eller and Gally. they decided to keep it that way.
Galchenyuk - Eller - Gallagher played together for all of 4 games in 2012-2013 for a total of 42 minutes.
http://www.cs.unb.ca/~mwf/habs/2012-13/regseason/season.html
Game 1 they had Galchenyuk on Plekanec's wing, after that they played him at center on the 3rd line with Prust and Gallagher. That line was actually killing it, they were producing great and because they were sheltered they were fine defensively.
That's actually how they should have left it. Galchenyuk on the 3rd line, in a sheltered role. Eller was breaking out offensively on the 4th line (Similar to Danault last season) and so deserved a shot to see what he could do, Plekanec was playing well but Desharnais was struggling. They should have right then and there gone with Plekanec-Eller-Galchenyuk down the middle. Instead they rearranged everything to try and get Desharnais going and gave him a mid-season extension despite his struggles.
It was only in 2013-2014 when the EGG line played a significant time together, and the same thing happened, Desharnais was struggling so the whole lineup was rearranged to get him going.
The number one reason Galchenyuk wasn't used at center is because Therrien thought Desharnais was one of the teams best offensive players, and he couldn't shelter both Desharnais and Galchenyuk and he went with Desharnais. We are likely seeing the same thing again this year with Julien, he doesn't want both Drouin and Galchenyuk at center and he's going with Drouin instead of Galchenyuk. Hopefully it will turn out better then it did with Desharnais.