I don't think Wells is going to blow anyone away in the long-term. He is having some success right now, but once the league adjusts to him, I think he will become a very average pitcher. He is 27 years old and his minor league numbers are not overwhelming, considering his age (he has never had an ERA under 4 at the AAA level).
Lou needs another lefty in the bullpen and is wondering if he can replace Marshall in the rotation, I'm betting Wells gets just a few more looks before things start catching up with him, but he has given the Cubs a decent run up to this point. Marshall is the superior pitcher, but until they decide what they are going to do with the bullpen, Marshall's situation will remain in question.
It would not surprise me to see them aquire a lefty and leave Marshall in the rotation, or acquire another starter to bump Marshall to the pen. They have money free in their preparation to trade for Peavy which obviously fell through so it wouldn't surprise me to see them make a move to clear up the situation.
Just my take on the situation, provides little answer though.