Zebo has been great, but he's not at Sexton's level - Schmidt
In committing his future to Racing 92, the Munster star was effectively ruling himself out of the international reckoning in the same way Marty Moore, Ian Madigan and Donnacha Ryan have done before him.
The only overseas-based player Schmidt has routinely selected was Johnny Sexton who was Ireland's first-choice out-half during the two seasons he spent in Paris from 2014-2016.
A week on from his squad being announced, the Ireland coach has expanded on his decision - outlining the unique circumstances that led to Sexton being allowed to circumvent the unwritten rule.
Playing in France took a toll on the Leinster out-half's body and Schmidt points to the wear and tear he has suffered since being asked to perform week in, week out in the Top 14 when considering his decision not to select subsequent departees.
Fundamentally, however, the reality was that Sexton was by far and away Ireland's best out-half during 2014-'16 and Schmidt had little choice but to select him.
Zebo has earned a regular spot in the starting XV over the past season, but does not have the same peerless status.
"At the time, I was new into the job," Schmidt said of his decision to pick Sexton when he was based in France. "To set a rugby team up in the space of a week is an incredibly difficult job.
"To have a thought process that is very similar to the guy that was running the team was, as much as anything, a real convenience to me going into a new job. That was a part of Johnny's selection.
"He is also proven to be, over the last two Lions tours, the starting player in a Lions Test match. Zeebs has been great for us. But, he hasn't reached the same level of selection, in those terms.
"I guess the risk is a really high-profile player does go and then you've got to make a decision.
"Since Johnny left, Simon would be as high profile as anyone has been. That's why it is tough for us and it's tough for people to accept.
"I do accept that people are disappointed because I am amongst those people. Is it emotive? It is for me because I've got massive respect for Simon, not just as a rugby player. He is an incredibly likeable young man. It is difficult for me as well.
"But, I have a responsibility to try to think as broadly as I can and try to future-proof as best I can what we think the picture will be over the next two, over the next six years and the risks inherent in selecting people who have signed elsewhere.
"It's a balance. I don't think there has to be a massive injury crisis at all. Form will still be a pretty big dictator come the Six Nations."
Speaking at the Guinness Storehouse ahead of the Rugby Writers of the Ireland awards this week, Schmidt insisted there is no firm policy around selection a la Wales, Australia and South Africa who require overseas-based players to have won 60 or more caps in order to be selected.
"There is no policy," he said.
"There is only an intention from the IRFU to best protect the provinces and the local game. We believe that the best way to do that is to select from within Ireland.
"The other real advantage is that selecting within Ireland, the players can be better managed. We already know that.
"There's one player (Sexton) who went to Racing 92 and was played for the first 12 games in the season, and I'm not sure he's ever had the same resilience since then.
"So we're looking to manage our players, we're looking to add to the longevity of our players. At the same time we're trying to support the provinces.
"The status quo has actually served us really well. But the world is always changing, so you've got to be always looking to adapt and there's never actually been a rule, so the flexibility is always there.
"The best comparison for us in terms of a similar-size country is Wales, but rugby is the national sport there and it is steeped in tradition in Wales that is embedded in the communities there. And you see the number of their high-profile players that have leaked.
"It's more than a trickle there, and we're trying our best to try to avoid that happening."
Probed on Sexton's injury profile since he returned from France, Schmidt said the toll of playing in the Top 14 has affected the 32-year-old
"It's pretty self evident," he said.
"He hasn't played 12 games in a row since then and that's because: you pick up the wear and tear and it's hard then to get that back. The wear and tear is something that, you can get a long rest and bounce back, and Johnny's had a few rests and he has bounced back.
"What you tend to do as a more experienced player, is you tend to manage your time a little bit better. But in this country you get that opportunity a lot more.
"It was the same with Paul O'Connell. I think he really extended his career by staying. He got himself in the right shape, his time was managed and there are players who have taken up that opportunity."
Schmidt has also rushed to the defence of players who have qualified to play for Ireland on residency grounds.
Bundee Aki is the latest import to be selected for the national squad having spent three years with Connacht and he is likely to be handed his debut against South Africa next week. Juxtaposed with Zebo's omission, the Aucklander's inclusion has come under scrutiny but Schmidt says he has not experienced the disquiet.
"Not really. I walk the streets, I meet people, people are incredibly supportive," he said. "If that was the case we wouldn't have two sold-out matches and one on the very cusp of it.
"When you put it into context, people like to focus on the very small margin and make that into a lot bigger than it is, but so far the players who qualify to play for Ireland make up 'round about six or seven per cent of the players who we have selected over the past four years.
"So I don't really see it as something that is as emotional or contentious as some people would build it up to be, but then I don't have access to everybody and there will be some people who will say that it is emotive and feel strongly about it.
"There are other people who would say that in the past we've had people who have played for Ireland who never, ever lived in Ireland, who had a grandmother or a grandparent of some sort who was Irish and therefore they played for Ireland but who never, ever invested in the local community.
"I don't think I've seen someone invest more in the local community than some of the people who have qualified to play.
"It may become a moot point with the change to five years to try to qualify to play for another country. The average professional career is six years, so there are a lot that are a lot shorter than that and there are a few that are quite a bit longer than that.
"If you estimate that it's six years and that it takes you three years to qualify for a country, that's half your career waiting to play somewhere.
"So I think it's never going to be perfect and it's hard for me to say that it's perfect, because I qualify as an Irish citizen, I've lived here for seven years now - well, a total amount of nine years - and we love the place.
"Does that mean we're any less Irish? It probably does because my accent is a bit weird! But I still think we've tried to invest in our local community as best as we can and our kids are very much invested in what they're doing at university and secondary school and I see very similar things happening with some of the players we have qualified.
"Certainly with Simon it's nothing against anything that he's done, other than that he won't be directly in our line of sight.
"It is a precedent that is always dangerous, should we select from outside of Ireland, because then if there are three or four other players who have substantially greater offers from somewhere else but are desperately keen to play for their country, then they may perceive that they can do both, and have both, and who loses then?"