The 2024 Masters champion presented McIlroy with his Green Jacket following his dramatic Augusta victory and could sense the Northern Irish golfer's "relief" at having ended his wait to complete the full set of major wins.
Scheffler said: "It was cool to see him get the job done, really cool to be there in that moment and to congratulate him and just see the joy on him and his family.
"He has accomplished everything in the game of golf and that was really the last thing to accomplish.
"The guy has won FedExCup, The Players, all four majors. Maybe the only other thing would be the Olympics he would want to win.
"It was such an emotional day for him. I just tried to stay out of the way and get the jacket on without embarrassing him or myself. I got to watch the whole celebration."
The world number one has won two Masters titles so far in his career but admits that he couldn't relate to the pressure McIlroy faced to complete the Grand Slam at the tournament.
Scheffler said: "I don't have the understanding of what it's like to be asked about the career Grand Slam but I have a small understanding of what it's like to be asked: 'Hey, you accomplished this, but you haven't accomplished that.'
"It can be very taxing on people sometimes. Definitely from the outside it looked a lot more like relief than anything [for Rory]."
The US golfer insisted that he is concentrating on "getting the most out of myself" when quizzed if he would take inspiration from McIlroy's achievement.
He said: "The feeling you get from winning a tournament is pretty special. For me, I wish it lasted longer than it did. It doesn't really last very long.
"Was it pretty awesome watching Rory win the Grand Slam? Of course it was. But as far as my life goes, I try to stay in the present. I don't think very often about career goals.
"Winning the career Grand Slam, I think that's something that any golfer would dream of, but at the end of the day, I'm just trying to get the most out of myself. That's about it."