That's according to former Gunners captain Martin Keown.
Sanchez suffered a bleak period at Old Trafford, scoring just three goals in 21 games.
The Chilean has already tired of the regime of Jose Mourinho in Manchester, tired of the defensive strategies of the Portuguese chief.
The Times wrote that the 29-year-old is bothered by Mourinho's focus on tracking, rather than attacking fluency.
Sanchez was in 9th place in the absence of Romelu Lukaku and showed better performance levels but was eliminated in Manchester derby, which United lost 3-1.
The player, who earns £ 350,000 a week at United, is allegedly angered by the omission of the first team and criticized the defense of his teammates after the game.
It has been a rapid decline in the form of Sanchez, who averaged just one league goal every two games in 122 games for Arsenal.
The Chilean endured some tough times in north London, however, sometimes digesting the frustration of the fans' growing anger.
Former Gunner Keown believes that Arsenal handled the best striker on such occasions, however.
"There were similar problems with Sanchez after he joined Arsenal, but they were resolved much more quickly," Keown told the Daily Mail.
"He started right but his Arsenal career took off as soon as he started playing left.
"After that, he seemed to thrive wherever he touched."