The Friuli outfit was beaten 4-0 in Spain and that result left Udinese level on seven points with Atletico Madrid at the top of Group I of the Europa League, but Celtic are now just two points behind them.
"We showed our inexperience and we were naive," Guidolin said. "We were rightly punished for our mistakes.
"But the group is still wide open and we will do our best in our next two games.
"My players were disappointed with the performance and I know they will be eager to make amends on Sunday."
Udinese return to the domestic competition looking to keep their perfect record at the Stadio Friuli.
Last weekend's 1-0 triumph over Palermo was their fourth straight victory in Udine and lifted Guidolin's side to second in the standings, just one point behind league leaders Juventus.
The hosts will be bolstered by the return of Antonio Di Natale, Mauricio Isla, Giampiero Pinzi and Gabriel Torje to the starting XI.
All four players rested in midweek with Guidolin having opted to field a second-string side against Atletico.
Rivals Siena travel to Udine inspired by last weekend's 4-1 triumph over Chievo.
That victory extended the Tuscan outfit's unbeaten run to four games and lifted them to seventh place.
"This is the Siena I want to see," coach Giuseppe Sannino said. "This is how we must play every game.
"I'm really happy to see that the team is growing game by game.
"We have to keep working hard knowing that every encounter will be a test for us."