Arda Turan scored the only goal of the match as Atletico assured themselves of a place in the knockout stages, with still one more match to play in the group phase.
The qualification comes as a relief to Atletico, who are 10th in La Liga, and Diego is confident the result in Glasgow will inspire his side to bounce back from their negative domestic results.
"We achieved our goal today, so I am very happy," Diego told Cuatro. "This is a very important victory, which will allow us to gain trust. We are growing.
"We met a very strong side at home today."
Atletico will host Rennes in the last group match, while Celtic will visit Udinese.
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