The 24-year-old was expected to arrive at the Madejski Stadium in December but new visa rules could result in the Samoan-born centre failing to join the Exiles until some time next year.
Despite the complications, Smith still fully expects Sa to become a London Irish player.
"There are some issues that we are going to be able to control and some we aren't," Smith told BBC Berkshire.
"He has his commitments with the Catalan Dragons until November. We weren't expecting him until December at any rate. There are one of two complications in terms of new government regulations that have turned things on their head over the past six months or so.
"I am confident it will happen. It's just a matter of timing. Best case scenario he is here with us by Christmas, worst case scenario we jump through the new hoops and regulations and he will be with us for next year at some point. That could be later in the season or the start of next but I'm very confident he will be with us."
Sa, who can play as a line-breaking centre or a back-rower, moved to Catalan Dragons from Sydney Roosters in 2009.