Leonid Slutsky said CSKA Moscow did not have the legs to defend their half-time lead against Manchester United.
CSKA and United played out a 1-1 draw in Champions League Group B after Anthony Martial secured a point for the visitors on Wednesday.
Slutsky and Co. took a 15th-minute lead through Seydou Doumbia, who was on hand to tuck away the rebound from Roman Eremenko's saved penalty.
Martial conceded the spot-kick following a handball inside the area but the France international redeemed himself with a 67th-minute equaliser against the fading Russians in Moscow.
"We showed very good quality in the first half. Things were working well for us and we might have gone even further in front," said Slutsky.
"After half-time, though, we didn't manage to play well enough on the counter attack.
"Looking at it, if you feel fatigue, mistakes happen. And given the current condition of Alan Dzagoev and Roman Eremenko, and Bibras Natcho's injury, we didn't have a player to dictate play in the middle of the park.
"We had to defend a lot and when you do that against a really good team then it's hard to avoid conceding goals.
"It's a shame but there are reasons why we couldn't continue playing at our best in the second half. A win would have given us a lot of cause for confidence in the group, but we still have a chance after this draw."
CSKA and United are level on four points, two adrift of leaders Wolfsburg.
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