After Herrera's transfer fell through on the final day of the transfer window in January, United came calling again in June and completed the signing of the Spanish midfielder in a big-money deal from Athletic Bilbao.
Herrera, who penned a four-year contract, has flourished in the United States since making the move to Manchester, making the midfield position his own under new manager Louis van Gaal.
Speaking ahead of Saturday's International Champions Cup fixture against Real Madrid in Michigan, veteran midfielder Fletcher told the club's official website the 24-year-old is set to make a big impact in the Premier League.
"He's a really talented footballer," Fletcher said.
"He has great enthusiasm and great energy in midfield. He always wants the ball and is always looking to make things happen.
"His enthusiasm's infectious - it's rubbed off on a few of us already - and he works hard for the team. He wants to learn and listen, he takes on instructions well… and off the pitch he's a good lad.
"He's settled in really well and not only with the other Spanish-speaking lads. I think he's going to be a key player for Manchester United."
While Fletcher is tipping the former Real Zaragoza man to make an instant impact, the Scot said the arrival of Herrera and Luke Shaw bodes well for the future at Old Trafford.
"First and foremost, they're both great lads. They're also top-quality players on the pitch," he said.
"There's no doubt in my mind they're great acquisitions – not just for the quality they bring to the team right now but for what they'll bring in the future.
"They're both young and should have many years ahead of them here. I'm sure they'll develop into real Manchester United players and help us win trophies."
