He stepped in only once previous year and got man of the match.
Toner will miss just his seventh Test match in the Joe Schmidt era, his near-constant presence since November 2013 underscoring his importance to Ireland's line-out solidity.
"So to lose a player like that, and the intellectual property he brings around the lineout".
"So it's a great opportunity to get him in and give him a run. So he'll be a loss, as any great player is".
When asked if Ireland's second-row strength turning to an injury glut is rotten luck, Best continued: "I suppose it's better happening now, and we'll see just how much depth we have".
Toner is Ireland's third frontline lock injury in this tournament, with British and Irish Lion Iain Henderson missing with a finger complaint and Munster powerhouse Tadhg Beirne out with knee trouble.
Ireland's depth at a position where they were thought to have a wealth of options at the start of the year has been severely tested but Best feels it could benefit them over the long term.
"When these things happen it gives an opportunity to somebody else", said Best. You need to make sure of the layer below that.
"And it's great to see him back involved".
'It does make it hard; the problem is he maybe could have played, but if you get another bump on it, it debilitates you quickly, and then you have to make another change'.
Replacements: S Cronin (Leinster), D Kilcoyne (Munster), A Porter (Leinster), U Dillane (Connacht), J Van Der Flier (Leinster), J Cooney (Ulster), J Carbery (Munster), J Larmour (Leinster).
Garry Ringrose's hamstring injury opens the door for Henshaw to go back into the centres, with coach Joe Schmidt understood to be ready to restore Kearney to the No.15 jersey.