As was discussed a few weeks ago, I think Mullin is more of a replacement for Mangan, a hardworking striker rather than a goalscorer. I think we will be chasing other, more prolific forwards as well.
That said, it is quite possible we won't be able to replace Cook. He came to us from a much smaller club at a time when he had not fully proved himself. Picking an established player like Cook straight off the shelf costs a fortune and might be beyond us, so we will probably need to get more out of the other goalscoring players we already have (eg. Jennings and Cole).