Roma were trailing 5-0 before an Edin Dzeko goal and Diego Perotti penalty dragged the side back into the tie.
Monchi lamented the result but said his side, who completed a comeback against Barcelona in the quarter-finals, had hope.
"It's a difficult moment, we were in form heading into this game and the result is not good," he said, via the club's website.
"But up until the final 10 minutes we were out of the competition. Now we still have some hopes, and we must take this small chance and do the same thing we did against Barcelona."
Monchi hopes his team can deliver at the Stadio Olimpico in the second leg next week.
"I always have faith in my team. It'll be difficult, we have done it once before and they will know that they cannot afford to underestimate us," he said.
"We have to have confidence and believe in it. The fans will be cheering us on – and 90 minutes at the Olimpico can be very long."