From your post, it seems like you're in a roto league. In a roto league, having to burn a roster spot on Morris if you don't have the DL slot to put him in is a real killer, and you're likely only to get a week or two of production out of him the rest of this year. Who is available on the wire that you could drop Morris for? If there's a solid starting pitcher out there, then I would do it and get production right now, but if you would be dropping Morris to take a flyer on a guy like Jose Lima, it's probably not a good idea.
As far as Sweeney goes, the last report that I heard is that he should be back in a couple weeks. With KC in the pennant race, and with them able to use him as a DH, you'd think he will be very motivated to come back ASAP. Yes, there is the possibility of him being done for the season, but there's also the possibility that he'll come back and be a very productive, top 10 1B in a couple weeks. There is more potential for production there than there is for Morris, who you'll be lucky to get 4 good starts from the rest of the year.