"I think Fernando helps Indy more than Formula 1. But I also see no disadvantage for F1," said Steiner, according to Speed Week.
"I also think it will be a one-off, because let's be honest - why would anyone want to miss Monaco? With Fernando, it's clear why he did it, but it would be more difficult for an Indycar driver to prove himself in Formula 1.
"And let's say Lewis Hamilton wanted to do it - ask Toto [Wolff] if he would agree."
However, it could be a different matter for McLaren drivers.
The British team's new executive Zak Brown hinted that McLaren might be considering a full-time Indycar foray.
"[Indycar] is something that we're definitely going to discuss and [we] have met with Indycar, and are certainly interested in competing in some way, shape or form in the not-too-distant future," he said.
Alonso has failed to pick up any points in 2017 after a frustrating start to the season at McLaren.