Alasdair Dickinson will replace Ryan in tomorrow's match in Durban.
Scotland head coach Scott Johnson added: “We are absolutely stoked for Ryan. To get another Scottish player, who we feel possesses the talent and resolve the play at the highest level – and playing for the Lions is the highest level – is great.
“People out there may be in for a pleasant surprise as to what a player he is.”
A replacement for Ryan is set to be announced by the Scotland camp later today.