 Eddie Patterson's Cliftonville are third in the league table |
Mark Holland's late winner at Mourneview Park means Cliftonville are virtually assured of third place. The Reds would have to lose their last three, and fourth-placed Portadown win their last three, to deny them.
If favourites Linfield retain the Irish Cup, third would give Cliftonville a place in next season's Uefa Cup.
Holland struck the winner in the 90th minute, taking Vincent Sweeney's pass and turning to shoot past Paul Rice from just inside the penalty area.
In the first half Glenavon's David Bracken had a good chance but Reds keeper John Connolly saved with his legs.
With Larne and Donegal Celtic winning, Glenavon are now involved in a three-way fight to avoid the relegation play-off place.
Larne occupy the one-from-bottom berth, but are just a point behind DC and Glenavon.