Transitions Need Objectivity
Many executives and managers in a leadership role think their job is to manage teams, work, and the organization through a transition. While this is true, a manager can not see through the eyes of other employees and may need assistance getting honest and objective perspectives. After 15 years in-house as an OD professional, I understand that hiring a consultant costs money, but I can also tell you that it's well worth the time you won't waste deciphering employee perspectives informally or through a survey that they are afraid to answer because they don't believe it's actually anonymous.
You can get more done and be a lot more productive if you allow for an independent third-party to help you through a transition by taking stock of the current situation, identifying challenges, and working toward goals regardless of organizational quirks. A person who is not your employee is also not overhead, concerned about losing a job, nor are they motivated by climbing a hierarchical ladder or a bonus at the end of the year. A consultant can help you clear the cobwebs and find out what people need, what drives them, and how to get them to buy-in to (and even lead) change. For short change and to mitigate productivity losses, isn't it worth the expense to get that objective perspective from a person who will work hard so that you can keep things moving forward?