Rucker is an undrafted rookie who signed with the Bears in April but was released earlier this month. He is 6-3 and posted some sparkling numbers at Memphis' pro day before the draft, including a 4.42 forty time and a 37.5 inch vertical leap. He did not get any playing time with the Bears so far in preseason.
Rucker has likely been brought in because the Jets were a man down at the receiver position with Titus Ryan having left the team earlier this week. It's probably too late for him to crack the rotation, which appears to be rounding into shape, but he has intriguing measurables, so could represent a good long term project.
Hilliard going onto IR is no surprise. That happens automatically whenever a player suffers a season ending injury.