He said the bungled stop - that Ricciardo claimed cost him victory in the race, won by world champion Lewis Hamilton - was caused by a delay to find the desired tyres trapped at the back of the garage.
Speaking to reporters after the race, Horner said they were simply unable to be ready for the Australian before he arrived in the pits. He apologised on behalf of the team, to the driver, for the blunder.
“Here in Monaco, the pit wall is upstairs and the garage is downstairs,” he said. “The tyres are on heat both in the garage and behind the garage - and, unfortunately, the set of tyres that were called for were not readily to hand.
“They were at the back of the garage. There was a scramble as the mechanics originally had the soft tyres ready - then, when a change to supersoft was requested, those tyres were actually right at the back of the garage.
“They couldn't be got to the car in time, which cost probably about 10 seconds in the stop.
“Despite that delay, they came out alongside each other.
“That showed how quick Daniel's in-lap had been and how slow Lewis' out-lap had been.
“So it was gutting for the whole team to lose a victory like that.”
Ricciardo dominated the opening laps of the race and was on course to win until the pit-stop muddle.
“As a team, we win together and we lose together,” said Horner.
“All we can do is apologise to Daniel and say that we haven't given him a good enough service, having done a great job with him to get the pole.
“He had done everything right in the race. Even going to the intermediates, it was the right way of going about things to get to the slicks.
“But, unfortunately, a communication error between the pit-wall upstairs and the tyre management (downstairs) let us down.
“Despite this, he had a really good go at attacking Lewis.
“He got pretty close, on the exit of the chicane, on one occasion, but just didn't quite have the opportunity to get past.” “We know how hard it is, in Monaco, to overtake and, effectively, the race was lost at that pit-stop.”
Original source: Red Bull boss shifts Monaco GP blame