Manchester United are "suffering" after six matches in 18 days, said boss Louis van Gaal following his side's 2-1 Champions League win over Wolfsburg.
Juan Mata's first-half penalty and Chris Smalling's 53rd-minute goal gave United their first Group B victory.
The Premier League leaders, who visit Arsenal on Sunday, had to hold on as Wolfsburg pressed for an equaliser.
"A lot of players are very tired because of the accumulation of a lot of matches," said Van Gaal.
|More from BBC Sport|
|Which two Man Utd players made Champions League team of the week?|
|Man City were lucky - Pellegrini|
|Ronaldo equals Real record in victory|
|Ronaldo's rivals: Weird & wonderful goalscoring records|
United lost their first group game against PSV Eindhoven, but victory over Wolfsburg means all four teams are level on three points after two matches.
"We have everything in our own hands and that is very important," said Van Gaal.
The Dutchman said his side showed "fantastic spirit" to recover from Daniel Caligiuri's fourth-minute goal.
"We score but after that it was suffering," he added.
"Arsenal played yesterday - one day more to recover. That will benefit them but we shall manage it."
- Relive Wednesday's Champions League action here
- 5 live's Football Daily podcast: Champions League reaction, analysis & debate
The problem? The 'rat race'
Wins for United and Manchester City on Wednesday ended a miserable run for English clubs in Europe.
With Arsenal and Chelsea beaten on Tuesday, the four Premier League teams had lost five of their six group games this season.
When asked what he thought the problem was, Van Gaal pointed to the competitive nature of the domestic league.
"We have a rat race that is not always the same in other countries," he said. "We also have to play over Christmas."
'I am the first'
Wayne Rooney has spent most of the past few seasons as United's regular penalty taker, a position now taken by Mata.
England striker Rooney has scored 24 of his 34 penalties for his club but missed from the spot during the Premier League game against Liverpool in March.
Mata said: "The manager made a list of penalty takers and I am the first right now, so I have confidence."
Van Gaal said: "I am responsible. Of course. I always communicate with my players. Everyone knows that."