Depending on your other RB's and the league setup. This just might be worth it.
I believe (and have some sound reasoning to believe so) that Morris will be the real deal.
I've been told (but not currently at 100% belief of) that Mendenhall will be limited throughout the season.
So, the questions are, how deep is your league? If you drop Mendenhall, will someone else scoop him up in 5 minutes? How much are you relying on Mendenhall to be there for you this year? If you have a relatively deep RB corps, and he'll still be there in a couple weeks if Morris falls flat, then why not?
"We couldn’t do diddley poo offensively, we couldn’t make a first down, we couldn’t run the ball, we didn’t try to run the ball, we couldn’t complete a pass - we sucked."