Jose Mourinho says Alexis Sanchez has already brought "maturity" to his Manchester United team and called him "an amazing addition" after watching the Chilean make his debut against Yeovil.
United won 4-0 at Huish Park to book their place in the fifth round of the FA Cup thanks to goals from Marcus Rashford, Ander Herrera, Jesse Lingard and Romelu Lukaku.
Sanchez, who was substituted after 72 minutes, had a hand in goals for Rashford and Herrera either side of half-time.
And it was his role in Herrera's goal just after the hour mark that particularly impressed Mourinho.
"Everybody is happy that he comes because good players they want good players and Alexis is a fantastic player for us," Mourinho told the BBC. "The question is always, which one is going to be out? Where is he going to play.
"But it's not about that for us. For us it's about having a fantastic group of attacking players and he's another one with a little bit more maturity.
"His choice for the counter-attack for the second goal is one that Marcus is still too nervous to make so he will bring us this maturity and class. We are pleased with him and I am happy with his performance."
Sanchez was lively throughout his 72 minutes on the pitch at Huish Park, and after his move from Arsenal was only confirmed four days ago, Mourinho was delighted to see him hit the ground running.
"I am very happy," the United manager said in his news conference. "It was no surprise for anyone. He is a fantastic player and has shown that already in three days training with us; he's an amazing addition for us."
Mourinho said his decision to field Sanchez from the start came from the forward's lack of recent match practice; he started for Arsenal against Chelsea on Jan. 3 but had only made one substitute appearance since.
"We spoke, he needed to play," Mourinho said. "He didn't play for Arsenal for three matches. I wanted him to go for 60 to 75 minutes and he was good."
Mourinho made 10 changes to his team for the visit to the League Two side. Only Juan Mata kept his place from the 1-0 win at Burnley on Saturday, while Michael Carrick made his first start since September and 17-year-old Angel Gomes came on to make just his second appearance with the first team.
United's overall performance pleased Mourinho, who said his players had been forewarned by facing the same opponents three years ago in the third round and also stated he had been impressed by the home side.
"It was a professional performance," he said. "They had the experience a couple of years ago and knew it was going to be difficult. If Yeovil scored first the situation would have been even more difficult.
"They were very well organised; they knew what they wanted to do on the pitch. I was listening to their manager, what he was asking their players to do, and he was very professional."
Despite the scoreline, Yeovil manager Darren Way was bursting with pride in his postmatch news conference, only regretting that his side could not have kept the scored level until half-time.
"I'm very proud," he said. "I thought for the first 20 minutes we caused Manchester United's back four problems. The game could have been different if we'd scored the two chances we had. If I had one disappointment it would probably be the goal they scored in the 40th minute.
"Playing against one of the best teams in the world, and best managers in the world, we basically lived the dream."