The Red Devils eased past Norwich City 4-0 at Old Trafford to get the Welshman's reign off to a winning start, but Giggs admits that it was difficult leaving certain players out.
"I didn't sleep last night [because of the team selection]," the 40-year-old, who took over from David Moyes earlier this week, is quoted as saying by ESPN.
"Players on the bench, leaving the likes of Juan Mata out of the starting XI was a difficult decision and a position I've never been put in before.
"It was really tough. I'm interim manager but I'm also one of their teammates, so it was tough to leave them out. But you've seen the response of Juan. That's a true professional, to not moan and not sulk and come on and perform like he did."
United remain seventh in the Premier League table following the victory.