There's a vein of popular thought that playing offensive line positions are mostly interchangeable, so that if you were a good left tackle then you should be a good guard. It's not quite that simple.
With a change of position comes different assignments to block, different footwork that you need to master, and all kinds of other changes. Last year, Rodger Saffold proved that he could play guard and tackle pretty well. All players aren't created equal, however, and the Rams 1st round pick Greg Robinson wasn't afraid to talk about his struggles with the position change so far with ESPN'S St. Louis Rams blogger Nick Wagoner recently:
It's frustrating that I'm not up to pace right now because the playbook is more intense and things are changing but once I get back to where I'm comfortable, things will calm down and I can be myself out there.
We can't forget that Robinson is a rookie and Saffold was a veteran when he changed positions. Also he wasn't in a pro style offense in college so it's not really a surprise that he's struggling early on. Still, Robinson was pretty open about what he has been going through:
It's just everything happens faster inside...Outside you have a little more time to kick. The guys are faster but these guys are just quick and trained to rush the passer. It's just about keeping my feet working and knowing when to pull.
Like the quote above, he mentions the difficulties of playing a new position. Every player can't just be plugged in to play anywhere. Sometimes they need time, and time is what Robinson will have. Honestly, I wouldn't worry too much about it. Rookie left tackles have struggled recently, so transitioning him to guard and then to tackle should make playing left tackle easier whenever he eventually becomes the "official" left tackle.
Let us not forget that the Rams didn't draft Robinson to become Orlando Pace in his rookie season. The Rams drafted Robinson in the first round because he has potential to be a great offensive linemen. Now to be clear, it wouldn't be a surprise if after his rookie season the Rams decide to place him at left tackle, but that's in the future.
If Robinson want's to be the Rams' franchise left tackle sooner than later, then becoming a better pass blocker will be the key thing for him. Until then, the Rams should have a hell of a run blocking guard.