Patinkin, if you'll remember, left the show because of "creative differences." Then, during the television critics association press tour, CBS programming chief Nina Tassler revealed that "creative differences" was an industry euphemism for "personal issues." Still no word on what "personal issues" is a euphemism for.
Anyway, Mantegna has been cast as FBI Special Agent David Rossi, an original member of the Behavioral Analysis Unit to which all the show's characters belong. Rossi is a retiree you has recently volunteered to return to the unit.
Reports from other media have stated that a number of big names -- including Harvey Keitel and Bob Hoskins -- were being considered to join up after Patinkin's departure, but I think it was smart to cast a capable, familiar but non-flashy actor like Mantegna. This is not a show that's about great acting. There aren't big dramatic scenes, no opportunities to show a lot of range. Maybe that's why Patinkin -- known for big performances on Broadway and in movies like "The Princess Bride" -- opted to leave. The show's real star is its formula: group of crime solvers untangles messy murders, assaults, etc.
Big name actors are out of place in this environment. Mantegna is a character actor, known mostly for his work in supporting roles: the dad in "Joan of Arcadia," Dean Martin in "The Rat Pack," and, of course, he's the voice of gangster Fat Tony on "The Simpsons." Mantegna isn't flashy, but he is a team player. He should blend in well. Good luck to him.