Vitor Pereira has admitted Nottingham Forest's European commitments are endangering their Premier League survival bid.
Forest remain just two points above the relegation zone after Sunday's 2-1 defeat at Brighton, which came three days after their Europa League play-off against Fenerbahce.
Pereira was able to rest captain Morgan Gibbs-White on Thursday night, and the midfielder scored Forest's equaliser after Diego Gomez put Brighton ahead.
But his tired team fell to a damaging defeat when Danny Welbeck struck Brighton's winner.
"They (Brighton) prepared in seven days, not in three days as we did," said Pereira.
"They tried to press us and we didn't find the proper spaces to go. They scored again after a mistake.
"We rested Morgan for the last game. It makes a big difference. If I didn't change for the last game, imagine now.
"Competing in the Europa League and competing again after three days, we needed to understand the proper times to press and the proper times to wait, the proper time to go fast or the proper time to keep the ball."
Forest travel to Manchester City on Wednesday before reigniting their Europa League efforts in the last-16 against Danish side Midtjylland.
The visit of the Superliga outfit will kick off a 10-day period in which Forest play four games, finishing off with a pivotal trip to fellow relegation candidates Tottenham.
Additional reporting from PA
