The season 10 contestant got engaged to Twilight star Reed, 23, earlier this year and it was assumed McDonald, 27, would perform. However, fellow Idol contestant Stefano Langone will also be singing.
Langone said to E! Online: 'I was in Salt Lake City and I played Nikki a song of mine, and she said: 'You need to play that at my wedding.'
'She absolutely loved it and I'm really excited to play for them. I love them both. They're great people and I can't wait to be there for them when they tie the knot.'