Alexis Sanchez's struggles at Manchester United have "no explanation", according to Ander Herrera, who says his old team-mate "had everything to succeed" at the club.
Sanchez joined United from Arsenal in a swap deal with Henrikh Mkhitaryan in January 2018, bringing to an end his hugely impressive four-year spell at Emirates Stadium.
The 31-year-old scored 80 goals in 166 appearances for the Gunners, becoming one of the finest imports in Premier League history in the process.
He was, however, unable to reproduce the same electrifying form at Old Trafford, and ended up being shipped out to Inter on loan last summer after netting just five goals in 45 outings across all competitions.
Sanchez is due to return to United at the end of the month, but it is uncertain whether he will figure in Ole Gunnar Solskjaer's plans.
Herrera played alongside the Chile international before his final contract at United expired, and admits he has been mystified by the mercurial performer's spectacular fall from grace.
The ex-United midfielder, who now plies his trade in Ligue 1 with Paris Saint-Germain, told The Athletic: "Sometimes, in football, there is no explanation for every single thing that happens.
"Alexis is one of them. He came from Arsenal. He used to win games by himself for Arsenal.
"I remember watching him because Arsenal were our rivals for titles and the top four. I saw them losing games 2-0 and Alexis would score two and they’d win the game. He’d score the winning goal.
“It shows football sometimes has no explanation. How can a player, who one month before, two months before, is the best player by far in a big team like Arsenal… then he comes to United and he doesn’t perform? I have no explanation."
Herrera added on Sanchez's failure to transfer his performances in training to matchdays at Old Trafford: "He trains good, he is a good professional, he tries to improve.
"In training sessions, you can see his quality. He scores a lot of goals. In front of the goalkeeper in training, he was lethal, scoring goals, goals, goals.
"He fights if he loses the ball, he runs back and wins the ball, so he had everything to succeed at United, and he didn’t do it.
"The only thing I can tell you is that I have no explanation."