Head coach Aitor Karanka says Middlesbrough showed "Champions League" quality in Saturday's FA Cup fourth round victory against Manchester City.
Boro defeated the Premier League champions on their own turf, thanks to goals from Patrick Bamford and Kike.
"Sometimes I say I have players who are playing in the Championship, who think they are Champions League players.
"They showed they are Champions League players, so I'm very proud," 41-year-old Karanka told BBC Tees.
"I don't have the words to explain my feelings. It's amazing the game, with our crowd and everybody. We have to enjoy this."
The Teessiders travelled to Manchester on the back of a six-match unbeaten run, and victories in their two previous matches.
Having repelled City's attack in a tight first-half, Boro took control after the break when Bamford finished off Albert Adomah's goalbound effort despite the attentions of Fernando.
"I told them at half-time, they could say I was crazy but we had Manchester City in our hands," Karanka said.
"The only chances they had in the first half were from our mistakes and in the second half we fixed those mistakes.
"We didn't lose the ball the same amount of time in the first half and we could create more chances.
"They trusted themselves and were thinking they could beat Manchester City."
About 5,000 Boro fans made the trip across the Pennines for the tie, their first visit to the Etihad for six years.
"It's my most emotional day, my proudest achievement," Karanka added.
"When I saw the draw my first thought was for the fans and I'm proud for them because they can enjoy this moment."