One can only assume Sanchez played a role in this. The team was likely able to get a lot of insider information on him. I am sure this was not the only criteria, but what Sanchez told them probably meshed with their own assessment. The fact the receiver already has good chemistry with the franchise quarterback is certainly a plus.
McKnight was considered too slow to play in the NFL, but he posted a surprising sub 4.5 40 during that pro day. He has good hands, and breaks jams and runs routes decently.
In the seventh round, you are probably ending up with a developmental guy. While a lot of guys might look attractive, everybody who made it to this point has real question marks, some of which only the teams know about. If the Jets are comfortable with McKnight, I think this is an ok pick. To be a realist, though, his only chance to make the roster is probably for the team to lose multiple receivers to free agency.
The Jets have kept alive their streak of picking players named McKnight who know Sanchez well. That makes two years in a row.