David Elebert's mistake just after the break, when he passed the ball across the box straight to Kris Commons, who drove past Tomas Cerny, was described as "suicidal" by the boss.
Reid said: "We had adapted well to the situation we were in.
"We set up really well and I don't think Cerny had touched the ball before Celtic scored with a fantastic strike from Kris Commons.
"I said to the players at half-time to continue to play in the right manner. But it was suicidal to give away the second goal. It was a disaster for us.
"David is gutted obviously. He did it in the first half and I had a go at him. But he gave it straight to Kris Commons who finished expertly."