The 24-year-old has reportedly been a target for Ole Gunnar Solskjaer's side all summer as part of the Norwegian's rebuild at the club.
Klopp had a front row seat to witness Sporting CP's Fernandes score and assist against his Liverpool side in a 2-2 friendly draw at Yankee Stadium on Wednesday.
And he was impressed by what he saw, insisting that, should United push through a deal to sign him, he would make their rivals stronger.
"If he [signs for Man Utd] then we will face him. He's obviously a really good player," Klopp said.
"They have already a few really good players, so that will probably make them stronger. It's not nice, but [Man Utd's transfer dealings are] not our cup of tea, to be honest."
Fernandes opened the scoring in the fourth minute against Liverpool, with his long-range shot forcing an error from Reds goalkeeper Simon Mignolet.
He then showed his ability to create, springing the offside trap and beating Trent Alexander-Arnold to set up team-mate Wendel with an expertly-played pass.
Fernandes caught the eye in the Primeira Liga last season with an incredible 20 goals and 13 assists as Sporting finished third and qualified for the Europa League.
Sporting president Frederico Varandas has said that any transfer fee to prise Fernandes away from the club will exceed €62 million (£56m/$70m).
The player himself has left the decision on his future to club authorities, but has admitted to wanting to play in the Premier League in the future.
"I don't care about the transfer market. I've talked about this a lot, I don't need to talk anymore. I already said I would like to play in England," Fernandes said.
"When the club decides [is when it will happen]. I'm not in charge."
Sporting coach Marcel Keizer also left the door open for his star man to leave following the match, claiming that the he would have to "wait and see" if Fernandes would be allowed to depart.
United have signed Daniel James and Aaron Wan-Bissaka thus far this summer, with Fernandes and Leicester City centre-back Harry Maguire having been strongly linked with moves.