Garcia is Europe's record points scorer with 28.5 from 10 appearances, Westwood played in 11 events, winning seven, while Poulter won five of his seven starts and was always considered one of the "beating hearts" of the European team.
All three are absent from Rome this year though after resigning their membership of the European Tour after defecting to the Saudi-back LIV golf setup.
Former vice-captains Martin Kaymer (38), Graeme McDowell (44) and Henrik Stenson (47) also moved to LIV and are not involved.
"It's certainly a little strange not having them around but I think this week of all weeks, it's going to hit home with them that they are not here, and I think they are going to miss being here more than we're missing them," McIlroy said on Wednesday.
"I think this week is a realisation that the decision that they made has led to not being a part of this week, and that's tough. The landscape in golf is ever-changing and more dynamic, and we'll see what happens and whether they will be part of it in the future. I always thought leading up to this week is when it's going to hit home that they are not going to be here."
McIlroy said that those changes meant that this has become "a bit of a transitional period" for Europe but that he had great faith in the rookies stepping up.
"I think the guys that we brought in are going to be awesome. Nicolai (Hojgaard), Ludvig (Aberg), Bob (MacIntyre), that's the future of our team and the future of the Ryder Cup," he said.
McIlroy stood firm against LIV from the start and now finds himself as the Europe's "Grandfather of the House" as he prepares for his seventh Ryder Cup.
With each one, however, he is uncomfortably reminded of his comment in 2009, before he had played, when he described it as an exhibition.
"I took a bit of grief for those comments and rightfully so," he said. "I think in 2009 I was just so focused on myself and trying to get my career off the ground that I felt like I had sort of bigger and better things to achieve for my individual goals and stuff like that.
"I just didn't put any emphasis on making a Ryder Cup team until you make one, and then you never want to be off one again.
"So I love being a part of this team. The most enjoyable moments in my career have been being a part of European Ryder Cup teams. I'm still very, very proud and probably proudest of the things I've done as an individual, but nothing - nothing - beats this week.
"It's an amazing experience and I want to be a part of it for as long as I can."
