The former Cliftonville man hobbled off after only 20 minutes and, in an ironic twist, it was his replacement, Junior, who lit the fuse for a sensational 13-minute four-goal avalanche.
Junior, formerly with Derry City, gobbled up a quick-fire double, sandwiched between goals from Aidan Wilson and skipper James Singleton.
Striker David Fisher finished the job early in the second-half with a stunning effort from distance.
Although Feeney was gushing with delight about the performance, he admitted Donnelly’s injury is a worry.
“Jay has been fantastic for me,” he said. “I still think he can be one of the best strikers in this league – he was just hitting form.
“It was a bad one for him, his ankle is not looking good. He’ll have a scan, as soon as possible. Fingers crossed for him.
“It’s just a pity because it was such a good performance, we had goals coming from all over the pitch – strikers and defenders.
"I was a little bit disappointed at the end...I thought we became a bit sloppy.
“You can have all the play, but it’s what you do in both boxes what’s important.
"I was pleased with the clean sheet, that’s two in a row, which is a big thing for me.
"With the players we’ve got, we should be more clinical and ruthless – it all fell into place for us.”
Feeney is hoping the performance sets the bar for the rest of the season.
“I said when I came to the club, that I’m not going to wave a magic wand,” he added. “The players are at one of the biggest clubs on the island.
"They’ve got to realise what we are and where we are. It’s a long, long season.
“I never made any predictions in terms of the league title, but if we can manage to nick a bit of silverware this season, I’ll be absolutely delighted.
“It’s a process, but if the boys can keep performing like that, I’ll be well happy.
"I thought we went about our job very professionally, that pleased me.
“With no disrespect to Ballymena, it could have been double figures.”
Angry Ballymena United chief Jim Ervin said his team simply ‘tossed in the towel’.
“We shipped four goals in the space of 13 minutes, which is unacceptable,” he fumed. “I couldn’t repeat what I told them at half-time.
"When the first goal went in our heads went down.
“We basically threw in the towel. As manager, the buck stops with me and it’s now up to me to change it.
"Come January, I know what I need to replace in this team.
“Yes, we are being punished for every individual error, but our boys didn’t half make it was easy for Glentoran.
“Yes, they showed a bit more fight in the second half, but it’s easy to do that when you are four-nil down, the game was as good as over. That fight should have been there from the start.”