If you can make this deal happen, do it. You can cross your fingers and hope the other owner doesn't know Matthews came into camp out of shape and unprepared. You can cross your fingers and hope he still has that waning hope for the potential of Matthews to pan out. While even a rejuvenated Olsen is not equal to Dallas Clark (when healthy, a beast), the upgrade from Matthews to Forte is much better than the downgrade from Clark to Olsen at a far less important position. Forte is a steady back, and should continue to produce, especially if you're expecting RB2 or RB3 numbers (depending on how you rank Blount, with AP as your obvious RB1) and if you can make this deal, it'll leave you in a strong position to start off the year.
That being said, I would consider using some of your RB depth to upgrade your WRs instead of trying to acquire another RB. While I love RBs, with Maclin being super iffy, MSW being in the uncertainty pit of STL, Boldin on the decline, and Stevie Johnson dealing with a weak offense and lack of another receiver, your WR corps has tons of question marks. If you could package Matthews and one of your WRs for an RB3 downgrade and maybe a Manningham type WR, I think you could do yourself well.
Regardless of what you do, good luck, and have fun out there.