I think the crux of the issue here is the difference between a 1st line centre and a centre who plays on their team's 1st line. There are not 30 of the former, whereas there are, at any point in the season, 30 of the latter - and that's the category that Grabovski would potentially fall in. He's conceivably capable of playing centre on his team's 1st line, but he's not a 1st line centre - the high end of his game hasn't shown itself to be high enough.