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1st round picks are notorious for arriving to camp late, often times severly late, as the organizations and the player's agent try to come to terms on a fair wage for the player. In Jenkins' case, I believe he will arrive on time, but you never know with some of the agents out there. In recent history, Reggie Bush was slightly late arriving to training camp and so was Robert Meachem, as their deals were delayed. Bush got away with it, Meachem suffered greatly. It is without question in Jenkins' best interest to be there on the very first day because there's a lot of depth at cornerback and a lot of solid players he'll be competing with for playing time.
Arnoux' case is a little trickier. The only player in recent Saints history who suffered a major season ending injury PRIOR to signing their rookie contract was receiver Chase Lyman in 2005. The Saints did the right thing back then and gave him a contract anyway, only to let him go at the start of the 2007 season (Lyman went on IR in 2006 as well suffering another ACL tear). All signs point towards the organization giving Arnoux a contract as well, but the team does have the option to not offer him a contract in which case Arnoux would be eligible to re-enter the draft next year. While an Achilles injury is a terrible one and is very difficult to come back from, Arnoux is still young and I'm sure the Saints will want him around for next year once he recovers.
Come back to Saints Nation for updates on the rookie contracts as soon as they become available.