Sanchez has struggled for form and fitness since arriving from rival Arsenal in January last year, with the 30-year-old scoring just five goals as a United player.

United caretaker manager Ole Gunnar Solskjaer said he cannot help Sanchez after the former Barcelona star's disappointing performance in last week's 2-0 first-leg loss to Paris Saint-Germain in the UEFA Champions League.

Ahead of Monday's (AEDT) FA Cup fifth-round meeting with Chelsea, Sanchez said he is determined to showcase his best at Old Trafford.

"He (Solskjaer) hasn't spoken to me a great deal about what I have to do on the pitch, but I think I'm an experienced player and I think I know what I need to do and what I shouldn't do," Sanchez said.

"I'm a player that, if I'm not in contact with the ball, I lose that spark, and sometimes I want to play in every game. You're in, you're out, and I'm used to playing.

"It's not an excuse because if I go on for 10, 20 minutes, I have to perform because that's what I'm here for, to make a difference.

"I would like to have brought more joy to the club. Yes, it worries me because I believe in my abilities as a player, I want to show it."

"I don't know how people see me, but football is what I've loved from being a kid and it's my passion," Sanchez added.

Sanchez also confirmed that he was injured after an assistant referee ran into him while warming up during United's UEFA Champions League fixture against Ligue 1 holder PSG.

The Chile international underwhelmed after replacing injured team-mate Jesse Lingard on the stroke of half-time.

"The referee's assistant, when we had a counter-attack, came flying down the line, and I had my foot there and he crashed into me. Bang," Sanchez said.

"I went back to the dugout and I said to [reserve goalkeeper] Sergio Romero: 'I've just been whacked and I'm about to go on'. I've got bruising but that's not an excuse for not being on my game in the way that I normally am."