Jose Mourinho is the world's best coach, not Pep Guardiola, according to Middlesbrough manager Aitor Karanka.
Mourinho and Guardiola arrived in Manchester ahead of this season, appointed at United and City, respectively.
Karanka, whose team face City in the FA Cup quarter-finals on Saturday, believes United picked up the better tactician.
"Pep is not the best coach in the world. The best coach in the world is Jose Mourinho," he told UK newspapers.
"He has shown it at Porto, at Inter Milan, Chelsea and Real Madrid and Manchester United. He was the best [in LaLiga] and he still is the best.
"I've never worked with Pep, but Jose started this season with some criticism, but he has already won two trophies for Man United.
"Jose is the best for me. I think Pep has to win the Premier League to prove [he is one of the best]. I think he has to win a trophy [this season]."
Mourinho has led United to Community Shield and EFL Cup success this season, while they are still in contention for the Europa League and FA Cup.
City are in the hunt for the FA Cup and Champions League, with Antonio Conte's Chelsea flying high atop the Premier League.
Karanka believes Guardiola, who had trophy-laden spells at Barcelona and Bayern Munich, has taken time to adjust in England.
"I think he's realised that this is a different football, a different league, and he has grasped how difficult the challenge is. It's taking him time to adapt and it's not been easy for him," he said.
"It's difficult when you arrive in a new country, with a new philosophy where a lot of things are different.
"He has some very good players, but the mentality here is different. Things are different and it takes time to adjust."