Red Bull lost vital running in each of the Friday sessions following a collision for Sergio Perez and a breakdown for Max Verstappen.
Horner admitted: “Mercedes have looked mighty in both sessions.
“I think we’ve had a sample of one track so far [this season] and they were every bit as quick, if not quicker in the [Bahrain] race, certainly better degradation in the race, and then they’ve been extremely strong here with both cars.
“We’ve got plenty on to try to give them a hard time.”
That leaves the team on the back foot ahead of a crucial qualifying session later today, where pole position or at least a grid spot ahead of Mercedes will be vital given the lack of overtaking opportunities at the Imola track.
“It’s going to be important because we know it’s hard to get close enough to pass around here,” said Horner. “Although the weather looks inclement over the next couple of days, even more so Sunday now…”
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