Thomas slid behind the favourites group with two kilometres remaining, after another leader Tejay van Garderen (BMC Racing) had already lost contact. Wilco Kelderman (LottoNL-Jumbo) took over the lead and Warren Barguil (Giant-Alpecin) and Andrew Talansky (Cannondale) moved into second and third overall, respectively.
Thomas, who finished second overall in 2015, slipped from third to eighth. He is 56 seconds behind Kelderman with three stages to race.
"In the last two kilometres of the climb to Amden, he didn't have it," Rasch told Cycling Weekly. "He was disappointed about the stage. He didn't have the legs. It's as simple as that.
"It's the weather, the rain, the fog, the cold... Those things add up racing over the big cols because he's super skinny now for the Tour de France."
Show us your scars: Geraint Thomas
The Tour de France starts in two week's time. If the weather is like last year, riders will be suffering under the French sun and not shivering down descents.
Rasch does not know Thomas’s weight, but the 30-year-old Welshman will have dropped a kilogramme or two in his preparation to be Sky's second leader at the Tour. This year, he switched from a Classics focus to aim for stage races and specifically, the Tour de France. This spring, he has already won the overall of Portugal's Volta ao Algarve and Paris-Nice.
Sky sent Chris Froome to the Critérium du Dauphiné last week and gave Thomas his freedom in Switzerland. It is his last stage race prior to the Tour de France. The race finishes at 2669 metres on Friday in Sölden, features a time trial on Saturday and ends with a mild stage on Sunday.
"He dealt with the same stage pretty well last year on the same climb to 2700 metres, he was good. We have to try to gain back time," added Rasch.
"He's 56 seconds back now, but a lot can change still. He's only 30 seconds and some to Talansky on the podium. Is the overall win still possible? They are both possible, the podium and the win. In the big mountains the situation can change just like that."
Thank you for reading 5 articles this month* Join now for unlimited access
Enjoy your first month for just £1 / $1 / €1
*Read 5 free articles per month without a subscription
Join now for unlimited access
Try first month for just £1 / $1 / €1