The Spanish midfielder joined the Red Devils for a fee in the region of £28 million from Athletic Bilbao this summer, and made his first appearance for the club in the preseason friendly against LA Galaxy.
Having played with Herrera in training and friendlies over the past month, Fletcher, United's longest-serving player, believes the 24-year-old will shine in the Premier League.
“He’s a really talented footballer,” Fletcher told the Manchester United website. “He has great enthusiasm and great energy in midfield, he always wants the ball and is always looking to make things happen.
“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.
“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."
One of the most difficult adjustments for foreign players coming to the Premier League is off the pitch. It is often tough for European players to settle in to life in England with a new squad away from their families, and for that reason Manchester United fans will be glad to here that Herrera is fitting in seamlessly.
United struggled mightily with ball retention in the ill-fated 2013-14 season under David Moyes, and Herrera was brought in because that is the exact area in which he excels.
Michael Carrick, perhaps the player most similar to Herrera in the United set-up, is out injured, leaving plenty of pressure on the Spaniard's shoulders to deliver straight away.
Herrera is pleased with the start he has made to life in the Premier League, however.
"I feel it has gone well for me so far," he said. "I'm happy with how I'm doing on the pitch and all the players are helping me in terms of integrating.
"My confidence is increasing with every game and I'm excited for the season ahead."