Taylor (left hamstring) and O'Kelly (abdominal muscle) are being assessed by the Lions medical team.
Coach Ian McGeechan, in charge of the Lions for Monday's game, said: "We are still awaiting the final assessment on both Malcolm and Simon.
"But with the close proximity of the game on Monday night, changes needed to be put in place so Donncha and Martin start with Ben and Lawrence coming onto the bench.
"This Lions party has the strength and depth to put in place changes such as these and we are looking forward to the first game of the 2005 tour."
Amended British & Irish Lions team to face Argentina in Cardiff on Monday, May 23:
Team: G Murphy; D Hickie, O Smith, G D'Arcy, S Williams; J Wilkinson, G Cooper; G Rowntree, S Byrne, J Hayes, D O'Callaghan, D Grewcock, M Corry, L Moody, M Owen (capt).
Replacements: S Thompson, B Kay, L Dallaglio, C Cusiter, R O'Gara, S Horgan.