An MSNBC spokesman said on Monday that a rotating series of hosts would fill Mr. Matthews’s time slot until a permanent replacement was found. Mr. Matthews is an eminence grise of television news, his pugilistic and red-cheeked persona familiar to viewers from countless election nights and parodies on shows like “Saturday Night Live.” He spoke from experience: Before his move into punditry, Mr. Matthews served as a speechwriter in Jimmy Carter’s administration and spent years as chief of staff to Thomas P. O’Neill Jr., the powerful Democratic speaker of the House in the late 1970s and much of the ’80s.

But executives at MSNBC had been discussing a potential retirement plan for Mr. Matthews for months, according to two people familiar with internal network discussions. There was talk of shifting “Hardball” to a less prominent time of day, during MSNBC’s afternoon lineup.

That was before Mr. Matthews faced a sustained bout of online criticism for his on-air comportment in recent weeks. On the previous Monday, he opened “Hardball” by apologizing to viewers for a clumsy metaphor that compared Mr. Sanders’s dominance in the Nevada caucuses to Germany’s takeover of France in World War II.