The Lurgan Blues suffered their fourth successive league defeat since McDonnell replaced Gary Hamilton last month - worryingly, it was their 10th loss in the current campaign where they have pocketed merely seven points from 13 starts.
Glentoran didn’t even have to get out of second gear to secure the 3-1 win – the game was done and dusted by half-time courtesy of two goals by Junior and Patrick McClean’s header seconds from the interval.
Glenavon did muster a token resistance after the restart but their only consolation arrived with practically the last kick of the game from a Jack Malone free-kick.
“I have to be brutally honest, the league table doesn’t lie,” said McDonnell. “Our performances haven’t been good enough and this was unacceptable.
“We haven’t won enough games of football, we haven’t scored enough goals, it’s as simple as that...we must find an answer soon.”
McDonnell did stress that he was missing a number of “experienced players” who would be crucial in times of need.
“We have a lot of key players out, including (Mark) Haughey, (Niall) Quinn and (Calum) Birney,” said McDonnell. “While we lost Sean Ward after only 30 minutes at the Oval…they are all leaders in the group.
“If we continue to lose games, we are going to have to look at the January transfer window to changes things.
"Although that is something I genuinely don’t want to do...but needs must sometimes, I must analyse the squad over the coming weeks.
"We are now coming into a run of fixtures against teams at the same levels as ourselves.
"We must now dust ourselves down and be ready to battle in every one of them.”
Although it was a stroll in the park for Glentoran, manager Warren Feeney was far from impressed by his team’s performance.
He said: “We should have been out of sight in the first half, but we didn’t take our chances.
"We need to get into habits of killing teams off...we must improve on our concentration levels.
“Yes, we were three goals up at half-time, it’s always a nice position to be in...but I told the boys it’s all about standards and being ruthless.
“We’ve got to be develop a killer instinct because, if we don’t, teams could come back to bite us.”
Feeney was also scathing over the way his team conceded in added time.
“The thing that hurt me most was giving up a goal in injury time, although it was a great free-kick,” he said. “But we should have nullified it further up the pitch and we didn’t.
“Glenavon did have a few chances in the second half...that’s why I’m so disappointed with our concentration levels.
"You can’t let teams back into a game, regardless of the score.
“We are still a work-in-progress...I’m still trying to find my best team.
"In saying that, I was pleased with the six points over the past two games.”