The 2016 Footballer of the Year dislocated his left shoulder during the first half of Mayo’s defeat to Tyrone in the league last Sunday after a heavy challenge from Padraig McNulty.
Keegan underwent an MRI scan earlier this week that revealed the full extent of the damage.
The four-time All Star is not expected to have completed his rehab in time for Mayo’s first championship match in less than eight weeks.
On a more positive note for the Westerners, Cillian O’Connor is expected to be available for selection for the May showdown with Galway at MacHale Park, Castlebar. The Ballintubber forward tore his hamstring in the early stages of last Sunday’s game, but he is expected to have recovered in time for the May 13 clash.
However, Mayo manager Stephen Rochford will have to plan without both for next Sunday’s league relegation battle against Donegal.
Meanwhile Conor McManus insists Monaghan’s small population or their poor results in Croke Park are not topics which dominate the players’ minds.
The praise for Malachy O’Rourke’s side for managing to stay in Division 1 for a fifth successive year, with the fourth smallest population in the country, bemused their two-time All Star forward.
Just as much as talk of their failure to win big in Croke Park, back in vogue ahead of Sunday’s trip to Dublin to face the All-Ireland champions.
“A lot is made of those things but we don’t read into any of those things. We don’t see it as a really big achievement staying in Division 1. Yes, it’s a target at the start of the year and we have done enough to stay there with a game to go so it’s a case of job done,” said McManus.
“We have a panel of 35, 36 players the same as every county, but there has been improvement from last year in that we have integrated new players. The likes of Davy Garland, Niall Kearns, Paraic McGurk, James Wilson and Aaron Lynch have all had game time.”