Ruben Amorim lamented Manchester United's bad luck against Aston Villa, claiming his side were "the better team" in their 2-1 defeat.
While United's attack has improved of late, their defensive frailties were on show once more; they have only kept one clean sheet in the Premier League this term.
It is their joint-fewest after 17 games of a top-flight campaign, along with 1961-62, 1959-60 and 1958-59.
United, who will sit seventh in the table for Christmas, are currently without a host of players, including Bryan Mbeumo and Amad Diallo, who are on international duty at the Africa Cup of Nations, but Amorim is not accepting excuses.
"We need to prepare all the guys that we have for the next game," he added.
"We cannot use anything as an excuse. No one is going to remember these problems, so let's cope with that. It will make us stronger.
"I saw from the beginning of the season a team that is going forward, but sometimes relax a little bit, and sometimes is not there in the right moment to fight.
"This was completely the opposite; we were the better team against a team that is strong at home and in a very good run. We were really unlucky."