United crashed out of the Champions League following Tuesday night’s 3-0 thrashing in the Nou Camp.
Lionel Messi scored twice while Philippe Coutinho added a stunning third to make it a humbling night for United.
But Ole Gunnar Solskjaer’s men made life even easier for Barca with some defensive mistakes.
Ashley Young gave possession away for the first goal before David De Gea allowed Messi’s second strike to go straight through him.
“Leo is Leo, we all know what we can do”
Sanchez, who made a second half substitute appearance, insists teams just can’t do this against the Spanish giants.
He said: “They played very well.
“We had very clear chances, but this is football, this is the Champions League.
“If you get something wrong or fail, then the other team gets on top of you. That’s what Barcelona do.”
Messi’s brilliance came as no surprise to Sanchez, who played alongside him at Barca before joining Arsenal.
The Argentine curled a delightful effort beyond De Gea to send Barca on their way and Sanchez was quick to laud his effort.
The Chilean added: “The truth is that the first goal was a very nice goal. Leo is Leo, we all know what we can do.
“It’s the same as always. Leo, he is very happy at Barca.
“He maintains this high level of performance and the team is also playing well.”
United are next in action on Sunday when they travel to Everton.