ATLANTA — San Francisco’s perfect start to the season was wiped out by a painful reminder that even an unbeaten team can’t overcome a 4-0 deficit in turnovers.

Former Washington Huskies star Michael Penix Jr. threw three touchdown passes, Atlanta scored 13 points off four turnovers and the Falcons handed the 49ers their first loss, 29-20 on Sunday.

“Any time you have four turnovers like that and you get zero, it’s real tough to win,” 49ers coach Kyle Shanahan said. “Too sloppy to win.”

San Francisco (6-1) was already thin at wide receiver before its offense faced additional limitations when running back Christian McCaffrey missed much of the first half with a thigh injury and didn’t assume his normal workload when he returned after halftime.

Rookie linebacker Kendal Daniels intercepted Brock Purdy’s pass intended for Jordan Watkins with 2:34 remaining. It was Purdy’s second interception.…