With Barca hosting Manchester United in the second leg of their Champions League semi-final on Tuesday, Valverde rested the likes of Lionel Messi and Sergio Busquets, while Luis Suarez and Gerard Pique were both suspended for the trip to Aragon.
Messi goes into the first leg of Barca's quarter-final against United having scored 10 goals in his past six appearances across all competitions, and has already struck eight times in Europe this season, too.
Defender Leonardo Bonucci was also rested along with Blaise Matuidi, Alex Sandro, Mario Mandzukic and Miralem Pjanic, with forward Federico Bernardeschi coming off the bench late in the game.
Kean opened after half an hour with Joao Cancelo pushing through the right with his shot going wide before the teenager stuck out his foot to redirect the ball home.
Mattia Perin replaced Wojciech Szczesny in the Juventus goal with Kean getting his second start alongside Paulo Dybala up front. He also handed defender Paolo Gozzi, 17, his Serie A debut.
And while we're all well aware of the magic Messi is capable of, he proved his toughness with his determination to play on after losing quite a lot of blood. "I regret this defeat because I think the match deserved more a draw than a loss".
Udinese remained four points above the bottom three.
It was a first top-flight win over Juventus for Spal which has gone through bankruptcy and been refounded twice - in 2005 and 2012.
Kevin-Prince Boateng shot over as Barcelona began to take control, but clear chances were few and far between in the opening half-hour.
Roma has a crucial match next week as it travels to third-placed Inter Milan.
The price was two points dropped and Atletico capitalised, beating Celta Vigo 2-0 thanks to a thundering Antoine Griezmann free-kick and a cool finish from Alvaro Morata. "The Champions League is everyone's dream".