Lainer played all four games for Austria at the last Euros, scoring once, but the Borussia Monchengladbach man was one of three players cut from the preliminary squad, along with goalkeeper Tobias Lawal and midfielder Thierno Ballo.
"It's a decision that wasn't easy for me. It was important to me to explain my motives to all three players in one-on-one conversations," Rangnick said.
"That's what I did today after the final training session."
Austria were already without the injured Real Madrid defender David Alaba, who will travel with the squad, as well as midfielder Xaver Schlager and keeper Alexander Schlager, also through injury.
Rangnick's side extended their winning run to six games with their 2-1 win over Serbia on Tuesday, where Patrick Wimmer scored his first international goal and have a final friendly match against Switzerland on Saturday.
Borussia Dortmund midfielder Marcel Sabitzer, who played in the Champions League final defeat by Real Madrid, is in the Euro squad, but will again be rested for the last warm-up game.
"We don't want to take any unnecessary risks so close to the European Championship. We need a rested and fit Marcel in Germany," the manager said.
Austria arrive at their training base in Berlin on Wednesday and their opening game at Euro 2024 is against France in Dusseldorf on June 1st. They will then take on Poland and the Netherlands in Group D.
Austria squad for Euro 2024:
Goalkeepers: Niklas Hedl, Heinz Lindner, Patrick Pentz
Defenders: Flavius Daniliuc, Kevin Danso, Philipp Lienhart, Phillipp Mwene, Stefan Posch, Leopold Querfeld, Gernot Trauner, Maximilian Wober
Midfielders: Christoph Baumgartner, Florian Grillitsch, Marco Grull, Florian Kainz, Konrad Laimer, Alexander Prass, Marcel Sabitzer, Romano Schmid, Matthias Seidl, Nicolas Seiwald, Patrick Wimmer
Forwards: Marko Arnautovic, Maximilian Entrup, Michael Gregoritsch, Andreas Weimann
