IMO, fixing defense against line rushes is easily fixable. Where offensively inclined Dmen get in trouble is that they often try to steal pucks and start a rush the other way, which works because their talent is so much better than those that they are playing against. All coaches have to do is emphasize that the main thing to do is defend the rush, and let the transition take care of itself. Yak has the reach and heft to be able to angle out most players, and a good or better stick if he puts his mind to using it for defending.
Bottom line is that he will be fine, especially if the Sens help him work on this aspect of his game. Working on his transitions should also help his puck retrievals, allowing him to get to pucks quicker and use his considerable Puck moving skills to get the puck out of his zone. His D zone coverage is effective, competitive, and fierce. That's a nice thing to have. Some D men just never have that aspect in their game.
For him, it's really about keeping it simple, and not trying to do too much.
I was hoping that MN would get a shot at Yak at #13. A big, mean'ish, skilled, RHD is hard not to like. I think his defensive faults are overblown, and part of what you get with an offensively minded, creative player. They simply take more risks than average players who are afraid to make mistakes.