IIRC, Karras has graded out well in his limited action over the past few seasons. He never was an issue and the o-line didn't miss a beat when Karras played in lieu of Andrews.
That's not to say that Andrews isn't a very good player -- he is. But Karras is a more than capable backup, and as others have noted, he's bigger, stronger, and more physical/violent than Andrews.
Andrews is more of a tactician with good feet and hands, uses leverage well, is good blocking in the open field, and perhaps most importantly of all, Brady/BB/JMcD trust him with line-calls, handing off blocks, adjusting on the fly, and so forth.
I think Karras can reach that point with more experience and playing time, and no doubt the team will work hard to get him prepared to perform at that level mentally -- physically there's nothing to teach, the guy is a beast.
And as another poster noted, I'm excited about the idea of having a big mauler at C for Michel to run behind.
I feel really bad for Andrews personally and there may be some growing pains with Karras, but I expect said growing pains to be largely unnoticeable and not affect the performance of the team.