Arsenal boss Arsene Wenger revealed he was surprised after Monaco allowed striker Anthony Martial to leave during the summer transfer window.
The 19-year-old joined the Red Devils for £36m but his fee could rise to £58m.
He enjoyed a very positive start to the new campaign, scoring four goals in all competitions.
"He was more of a winger in his [early] career," Wenger told reporters on Friday.
"He played the final part of last season as a striker. Monaco's statement was always that two players, Martial and Layvin Kurzawa, would not move.
"I thought they would maintain that but in the end both of them moved.
"It's [his season start s] a surprise because sometimes it's quite difficult at the start.
"To maintain that will be very difficult for him so he will have to have some understanding of that as well if he starts to score less. He has already set the expectation level very high.
"I'm not surprised that he's done well because he's a very talented player."