McDermott Will & Emery LLP has named John M. Callahan, who heads the firm’s international health merger and acquisitions practice, as the new managing partner in Chicago.Callahan replaces Lydia R.B. Kelley, who will provide guidance on office administration in her new role as the operations partner.“I think I have some big shoes to fill,” Callahan said of his predecessor.Callahan, who has spent nearly 20 years at McDermott, advises clients on health sector mergers and acquisitions. His clients include …