The signing of Chris Johnson means the team is currently looking at a tandem of Johnson and Chris Ivory. Do not take this to mean the Jets now are guaranteed to pass on running backs in next month's Draft. A position does not have to be a hole for a team to invest in a position.
Now I don't think the Jets are going to use one of their top two picks on a back because there is arguably no player in this class who would merit it. This is a good spot to be heading into the Draft, though. If a player the Jets like at the position is available, they can grab him. Johnson and Ivory are locks to be on the roster, but Bilal Powell and Mike Goodson don't. A talented young back would make either expendable. There wouldn't be any pressure for the rookie to play right away either. If he is ready to contribute, he could play his way into the lineup, but Johnson and Ivory could play as planned if he wasn't ready.
I just wish the Jets had gotten themselves to this spot at more positions. There are some good looking backs who should be available on the third day, though. Chris Johnson shouldn't stop the team from investing in one if the value is right.