Lions Tour 2005: Saturday, June 4Bay of Plenty v Lions, Rotorua International Stadium, 8.10am Irish time (live Sky Sports 1, RTE Radio 1)
O'Kelly's injury, picked up whilst training at the Lions' Welsh base in Glamorgan, was troubling him at today's session in Auckland and as a precautionary measure, the 26-year-old O'Callaghan is in, and in line for some involvement against the Steamers.
Coach Sir Clive Woodward said: "It is obviously disappointing for Malcolm but this is why we have the strength in depth of the touring squad.
"It enables us to provide cover to ensure that we get the players back to the full fitness needed to tackle the tough games ahead."
Nonetheless, three Ireland players will start the Lions tour opener - captain Brian O'Driscoll, fly-half Ronan O'Gara and second row Paul O'Connell.
Leinster and Ireland talisman O'Driscoll is set to skipper the 2005 Lions for the first time, in his seventh outing in the famous red jersey.
The Dubliner, who made his Lions debut against Western Province in June 2001, said: "After all the waiting and the preparation it will be great to get out there and play the first match in New Zealand.
"This is a strong side and we can't wait to get out in front of the New Zealand public and show what we can do on the rugby field."
O'Driscoll will be partnered by Wales star Gavin Henson, with a number of favourites for Test places included in the backline, namely Josh Lewsey, at full-back, and Welsh scrum-half Dwayne Peel.
O'Gara is also given a timely boost and chance to impress as Peel's halfback partner. The 27-year-old Corkman started the Lions 2001 tour opener against Western Australia in Perth, kicking 13 conversions out of 18 in a record 116-10 victory.
O'Gara's Munster colleague O'Connell, 25, cannot wait to step out for his Lions debut.
"It will be a tremendous honour for me to pull on a Lions jersey for the first time. It's really exciting to play alongside players of such quality like Lawrence Dallaglio and Martyn Williams.
"I will be playing with players who I have admired for such a long time," added the 6ft 6in lock.
"It will be a great experience and an important test for both me and for the team."
While there will be a consultation on O'Kelly's condition tomorrow, the lock's Leinster and Ireland colleague Gordon D'Arcy is the other Irishman on the replacements bench.
Bay of Plenty: Adrian Cashmore; Filimone Bolavucu, Allan Bunting, Grant McQuoid, Anthony Tahana; Murray Williams, Kevin Senio; Simms Davison, Aleki Lutui, Ben Castle, Mark Sorenson, Bernie Upton, Wayne Ormond (Capt), Nili Latu, Colin Bourke.
Replacements: John Pareanga, Taufa'ao Filise, Paul Tupai, Warren Smith, Charles Hubbard, Robert Moon, Apoua Stewart.
BRITISH & IRISH LIONS:
(15) Josh Lewsey (Wasps/England)(14) Mark Cueto (Sale Sharks/England)
(16) Steve Thompson (Northampton Saints/England)(17) Andrew Sheridan (Sale Sharks/England)(18) Donncha O'Callaghan (Munster/Ireland)(19) Martin Corry (Leicester/England)(20) Matt Dawson (Wasps/England)(21) Charlie Hodgson (Sale Sharks/England)(22) Gordon D'Arcy (Leinster/Ireland)
Referee: Paul Honiss (New Zealand)