Newcastle United fell behind inside four minutes at the Mestalla, Filip Ugrinic curling in the opener, and the contest turned feisty when Anthony Elanga was stretchered off following a poor tackle from José Gayà, who was sent off as a result.
Jaissle admitted the sight of Elanga in distress had rattled him, though the forward was soon back on his feet with what turned out to be nothing more serious than a bruised shin. Substitute Yoane Wissa scored twice to complete the turnaround, while fellow new arrivals Bazoumana Touré and Sean Steur, signed this summer from Hoffenheim and Ajax respectively, also caught the eye after coming off the bench.
Despite the win, and Newcastle's first trophy of the Jaissle era, the German head coach was careful to temper any celebration.
"We will not smash the locker room now and celebrate a lot," he said. "We still have a lot to work on."
He did, however, praise his players' response to a difficult opening spell in his first match in charge.
"Winning games always helps to gain confidence. We also knew that we had a hard week behind us. The players did well and there was a huge commitment to see in each session, so they really deserve two days off and then we start again."
On Elanga's injury, Jaissle was candid about his initial reaction. "Of course I was in fear, definitely, but now I'm more relaxed as the medical department is showing almost green lights, and there's nothing serious."
The match came at the end of a turbulent period of change at St James' Park, with Eddie Howe having departed as head coach and key players Alexander Isak, Bruno Guimarães, Sandro Tonali and Anthony Gordon all leaving the club within the last year.
The average age of the squad has fallen sharply as a result, with 28-year-old Harvey Barnes the oldest starter against Valencia, and goalkeeper Lukáš Horníček, 24, midfielder Aladji Bamba, 20, and forward Touré, 20, all making their debuts, while 18-year-old Steur impressed from the bench.
Jaissle has a track record of developing young talent from his time at RB Salzburg, and he made clear patience would be required as the new generation finds its feet.
"Time is the key," he said. "They're young lads. It's never going to be easy in football, especially at the highest level, but we're here to do that."
Jaissle also showed an early willingness to make tough calls, leaving goalkeeper Nick Pope, a key figure under Howe, out of the squad entirely.
"I had a talk with Nick," he said. "I really appreciate what he did in his career so far. He knows exactly the situation right now and we need to deal with it in the best possible way, but I always appreciate the player and the human being behind."
Newcastle are still in the market for further experience, particularly in central midfield, with Dan Burn and Fabian Schär expected to remain important dressing-room voices. Jaissle now travels to Newcastle for the first time as head coach ahead of his formal unveiling at St James' Park next week.
"For sure I will be visiting the stadium and getting my first impressions," he said.