I took a second look at Mumbai's bowling stocks, and bowling is their strength. That their batting is a weakness is an understatement, though- it does look a little weak bar Sachin, Sanath, Bravo, and- guess who- Uthappa. Even if they lose Nehra, they are still left with Dilhara (available often but subject to fitness), Malinga (subject to fitness), Nel (no problem), Zaheer (once the deal is settled), Harbhajan (tough to leave out), Kulkarni (decent IPL last time, fantastic Ranji season), Bravo (useful stock bowler), Nayar (under-rated seam-up bowler) and Smith (likewise), so that's not such a problem. Fitness issues of a few (mostly Lankans) will be a key. This makes the entry of Dhawan a good move for Mumbai.
Delhi have rather thin bowling stocks- except McGrath and Vettori, they don't have too many. Nehra will add some experience and limited-overs intelligence, as also Maharoof. Mishra is a dicey pick- he's the kind who'll get slaughtered, but he's also claimed a hat-trick. How many runs they squeeze out of Mishra, Maharoof and Vettori is a key factor in their performance- you drop one bowler and the part-timers will get smashed around.