Johnson has been on my "sell high" list before, and as long as he keeps scoring touchdowns like he did on Sunday he's probably going to stay here.
If you merely glance at the output, Johnson's 12 fantasy points last week look pretty solid. If you look at the fact that he had only three carries and six yards on those carries, not so much anymore. Johnson vultured a couple of goal-line touchdowns against the Lions, but wasn't a part of the offense otherwise and played a clear third fiddle to Chris Johnson and Andre Ellington.
Touchdowns have greatly inflated David's value, and inflation is a wonderful thing for owners looking to sell. He's a RB3 who belongs as depth on the bench. If you can pull a RB2 or sweeten another deal with him, go for it.