Rashford scored twice inside five minutes in the first half on Sunday - just three days after opening his United account with a double on his debut in the Europa League against FC Midtjylland.
And despite all the expensive signings on display, it was the 18-year-old who caught Wenger's eye in a damaging defeat for the Gunners.
"They had still a few millions on the pitch today, from [David] De Gea, to the midfield and even [Memphis] Depay," he told reporters.
"The player who was suprising for me was Rashford. The timing of his movement, [and] the intelligence of his movement was excellent."
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Arsenal responded to Rashford's double through a Danny Welbeck header, but fell further behind in the second half after Ander Herrera's shot deflected off Laurent Koscielny and beyond Petr Cech.
Mesut Ozil closed the gap but the Gunners could not find an equaliser and finish the weekend five points behind Premier League leaders Leicester City and three adrift of rivals Tottenham.
Asked what impact the defeat would have on the title race, Wenger responded: "Nobody knows. We dropped three important points today.
"We cannot feel sorry for ourselves. Mathematically what it means, we will know at the end of the season."