For John J. Mack, the new chief executive of Morgan Stanley, it was a moment to savor. On his triumphant return yesterday to the investment bank that unceremoniously dropped him four years ago, Mr. Mack was greeted by a standing ovation from a crowd of 200 employees gathered in a conference room at Morgan Stanley's towering Manhattan headquarters on Times Square. But for all the cheering, Mr. Mack immediately faced the task of bandaging the wounds that deeply scarred the fractured firm under Philip J. Purcell, the former chief executive of Morgan Stanley who forced out Mr. Mack in a power struggle in 2001. The tensions facing Mr. Mack were immediately evident in the announcement after the market closed that Zoe Cruz and Stephen S. Crawford, co-presidents of the investment bank under Mr. Purcell, had resigned from the board, the first of what are likely to be a series of high-level moves. Mr.