"We believe that Aon must carefully balance what could be an overwhelming pace of change to avoid unanticipated side effects," S&P said.

Fitch said it "believes in the long term, Aon's acquisition of Hewitt will result in positive business and operation synergies, with reasonable integration risk."

Hewitt is a world leader in human resources consulting and outsourcing, and the transaction "will significantly increase Aon's market share in this area," Fitch said.

The rating service went on to say that it believes the Aon management team "has a very good track record related to the execution of strategic plans and expense cutting," and for that reason the broker can manage the integration risk.

In an analyst's note from Deutsche Bank, the firm said it sees limited chance for competing bids for Hewitt. The most likely potential bidders are IBM, ADP and Accenture. It also expects no anti-trust regulatory issues.

Aon said yesterday that it plans to acquire the Lincolnshire, Ill.-based human resource and outsourcing consulting firm in a 50-50 cash and stock deal. The deal is expected to be completed by mid-November and the subsidiary will be named Aon Hewitt.

Yesterday, Moody's Investors Service affirmed Aon's debt rating, but changed the rating outlook from stable to negative. The rating service said the change was made over concerns with the move and the execution risks of such a transaction.