The Australian has found himself without a race seat this season off the back of three consecutive titles in Formula Renault, Formula 3 and Formula 2.
Winning the F2 title in his rookie season effectively left him with nowhere to go this year, with the Alpine seats filled by Esteban Ocon and Fernando Alonso.
That prompted the team to instead put Piastri in its test and reserve driver role.
However, Rossi is adamant that the year on the sidelines won't hamper Piastri's rapid development, with the team effectively redesigning the role around him.
That includes Piastri taking part in every test and simulator session, as well as playing an active role in briefings on race weekends.
"It's going to be an extensive testing programme, a lot of test days, a lot of kilometres, a lot of simulator," said Rossi, when asked by Autosport about the unique version of the role.
"Usually, we don't have the same drivers doing all of the test days or the simulator, only because it helps having different points of view.
"But we feel that Oscar can probably bring more than less seasoned drivers on the simulator, first of all. And will get more out of an extensive programme on track and off track in the simulator.
"He's going to learn a lot and bring a lot to us as well.
"Plus, we are including him a bit more in depth than other drivers in the briefing room of the data analysis.
"He's also looking at other cars and their driving lines and providing potential feedback on the way the cars behave, or even the drivers, how they take some turns and stuff like that.
"Oscar has better input, better feedback, because you can rely on it. It's not like someone is saying something, and you're like, 'yeah right, what do you know?'.
"So he's getting a lot of information, a lot of experience and expertise. It's much more intense than a rookie or reserve driver programme will [usually] get.
"Even compared to last year, my reserve driver was not necessarily involved in all those briefings or debriefs. Clearly not doing all of the tests. Clearly not going on the simulator so much. And that's normal.
"Oscar is doing it all."
There are also plans to have Piastri take part in free practice sessions later in the season.
According to Rossi, the team would have loved to have done so in Melbourne to make a splash for Piastri's home race, but the new regulations and new layout meant it couldn't happen.
"We loved the idea, I loved the idea," he said. "It would have been a great story.
"But, to be honest, those cars are brand new. They are still rather fragile for all teams. The drivers are still learning and we are still learning, so any FP, even FP1, is important.
"The track is new as well. So it was a perfect storm. And we don't have a lot of spare parts.
"It's one thing to be the regular driver and break the car, it's another thing to put that pressure on a rookie, as talented as he is. If suddenly you go and destroy the chassis, for any reason, then you are like Haas, now you don't have a spare chassis.
"There were too many parameters going against it. So sadly we decided it will be later in the season.
"We have a couple of grands prix that are earmarked as possible. Think about grands prix where the drivers are very comfortable because they've raced on that track like a billion times and FP1 is not that important. That could be an option. And/or grands prix where stakes are perhaps less high."
