Alexis Sanchez said he has found it difficult to adapt to Manchester United's style of play since arriving in January.
Sanchez struggled to find his best form after leaving Arsenal, scoring just three goals in 18 games.
Jose Mourinho was criticised by some United fans for what they perceived to be a cautious approach in the FA Cup final defeat to Chelsea on Saturday, and Sanchez said he has struggled with the way he has been asked to play since joining the club.
"I think that in every game I've played in up to now, I've maybe found it hard to adapt to the style of play and I've been getting to know my teammates," he said in the FA Cup matchday programme.
United finished 19 points adrift of champions Manchester City this season, despite beating them at the Etihad Stadium, and also defeated Liverpool, Tottenham Hotspur, Chelsea and Arsenal at Old Trafford.
But they dropped points against six of the bottom eight clubs and Sanchez said that will have to change next season if they are to mount a serious title challenge.
"I believe we need to improve in all aspects," Sanchez said. "We need to be focused and concentrated in the big games and the same in the so-called lesser games, too."
After defeat at Wembley, Sanchez is still waiting for his first trophy at Old Trafford.
United will be expected to take another step forward in the Premier League and the Champions League next season, and Sanchez insisted it is those competitions that a club of their size needs to be winning.
"I believe that United really is a very big club on a worldwide scale and even more so in England, where it's the biggest," he said.
"They always want to win and to be fighting on all fronts and that's the reason I wanted to come here. I also had some great players alongside me at Barcelona, some highly experienced guys who had great quality. I think that United are not too dissimilar as a club to them in their size and stature."