Blake actually played 17.1 mpg last year and he played in 56 games, starting 24. 20 games he played over 20 mpg. That is a fairly meaningful role.
I don't think it is unreasonable at all to think he could play 20+ mpg in about 2 out of 3 games for the first couple months of the season, and then scale his minutes down when Rob comes back.
That said, I'd still much rather have Howard, Whiteside, or Cousins. I think they have a lot more left in the tank and are all far more capable of starting for 2 months, while still producing at a decent clip, than Griffin is.