Arsenal manager Arsene Wenger defended his decision to replace Alex Oxlade-Chamberlain with Andrei Arshavin in the second half as the Gunners lost 2-1 against Manchester United.
Wenger's move to replace 18-year-old Oxlade-Chamberlain, who set up Arsenal's goal, with Arshavin at 1-1 drew an angry response from the Emirates Stadium crowd - and captain Robin van Persie.
But Wenger told Sky Sports 1: ''He started to stretch his calf and he had given a lot, he is not used to that intensity.''
Wenger continued his defence of the Oxlade-Chamberlain substitution in a frosty post-match press conference.
''I can understand that the fans are upset about the substitution, especially when it doesn't work, but he (Oxlade-Chamberlain) had started to fatigue,'' he said. ''He was sick in the week. Arshavin is captain of the Russia national team.
''You have an 18-year-old kid making his first Premier League start and a player who's captain of his country and they are querying the substitution? Let's be serious.
''I've been a manager for 30 years and have made 50,000 substitutions. I do not have to justify every decision I make to you (the Press). I stand up for it.''
However, Wenger admitted the result leaves his team in real danger of not making the top four this season.
''It leaves us in a very difficult position. It's a game we couldn't afford to lose,'' he said.