Ian Rapaport is one of the few national reporters with deep roots in the New England Patriots organization, and presumably those roots have a bit of carry over to "Patriots South" down in Houston. In short, when Rap says something, I listen.
Cassel could be classified as a "journeyman" by this point in his career, but he will always be remembered for his big break as a Patriot. That big break, however, came while Bill O'Brien was coaching wide receivers in New England, not quarterbacks. When you hear about Cassel coming from O'Brien's tutelage, just remember that neither of these men were even working in the same position group in 2008. When O'Brien was promoted to quarterbacks coach and offensive play-caller in 2009 to replace Josh McDaniels, Cassel was traded to the Chiefs, along with current Texans linebackers coach Mike Vrabel, for the 34th overall pick.
Since leaving the Josh McDaniels umbrella in New England, Cassel has largely been a pedestrian quarterback. The former Patriot, Chief, and Viking has posted only one season of more than 3,000 yards (3,116 to be exact) after 2008 and only two seasons in his entire career with 20 or more touchdowns (21 and 27, respectively). In 2012, Cassel lead an extremely talented Chiefs roster stocked with Pro Bowlers all the way to a 2-14 record and the first overall pick. If that sounds familiar, it's because it is.
I do not doubt that Houston is at least taking a look at Matt Cassel just for the sake of keeping all of their options open, but it is hard to imagine him excelling in any role that is not as a veteran backup.