James Tominks (shoulder), Enner Valencia (toe), Andy Carroll (knee), Winston Reid (hamstring) and Carlton Cole (knee) were all missing as goals from Olivier Giroud, Aaron Ramsey and Mathieu Flamini fired Arsenal to their fifth Premier League win in a row.
McDonald, who was standing in for Sam Allardyce in the post-match press briefings while the West Ham manager tried to refocus his team, also revealed that goalkeeper Adrian dislocated his finger in the warm-up but played through the pain barrier.
“When you are missing vital players through injury, to try to adjust and playing one of the best players in the league becomes very difficult,” he said.
“Even the goalkeeper has dislocated his finger before the game (in the warm-up), but Adrian has been a brave lad and played on with a very good performance, making some great saves.
???Then they probably had three chances in the second half and scored two, which is how clinical you have to be at this level.???
The defeat means West Ham have now gone eight games without a win in all competitions and are 10th in the Premier League table having been in fifth place on Boxing Day.
McDonald admits his team are struggling but puts that down to the fact they have played some of the higher placed teams in the Premier League recently.
???Football is all about results, and of course it is,” he said.??
“We???ve played a lot of the top teams recently and we???ve competed against them ??? yes, we???ve drawn games we should have won and lost some but these are difficult games.
???The manager has been really pleased with most of the performances but this game is all about results. Hopefully the next few games are a good set of games to put some points on the board and get the confidence back.
???We???ve competed against some of the best teams in the league, so let???s see how we can do in these next few games and put some big points on the board and get us back up the table to where we were before we played the top six.???