Texans need to tank for a QB, they have a lot of issues (including on defense) that people don't realize.
Here's the thing - O'Brien would probably like to have Hackenberg, but Hack's stock is in a complete nose dive. It might be hard for Texan ownership and FO to sign-off on Hack. And Hack might not even come out next draft overall.
Jared Goff is going to be the #1 QB selected and the Browns will take him. And maybe Connor Cook will be the #2 QB selected. So depending on where the Texans draft, Hack could be available to them if he comes out.