In a letter to councillors, Watson's chief of staff Serge Arpin said the mayor was injured during a community snowmobiling event in the rural community of Osgoode on Saturday afternoon.
Watson was taken to the Queensway-Carleton Hospital for treatment and released later on Saturday.
The letter states Watson will need between four and six weeks for a full recovery.
"It is likely that Mayor Watson will have to curtail his involvement in community events over the coming days and weeks," the letter said.
Final budget vote on March 11
This comes at a difficult time, as the city's budget faces a final council vote on March 11.
But it is not clear whether a temporary replacement will be named.
Ottawa last needed an acting mayor in 2009 when Larry O'Brien was forced to take a leave of absence.
Former councillors Doug Thompson and Michel Bellemare took turns replacing O'Brien over a four-month period.