Oxford defender Harry Worley was thrown up front with seven minutes to go and repaid boss Chris Wilder with a goal to cancel out Kevin Ellison's 78th-minute opener.
The Shrimps extended their unbeaten home record to nine games, but Bentley said: "It was two points dropped, definitely, and I am massively disappointed.
"Three points would have given us a gap over Oxford which is important in this league as it is going to be a scrap for all the teams in the lower half of the table in the second part of the season.
"After a slow start we got into the game, got a good goal from a set play, but one lapse of concentration cost us dearly and it isn't the first time that has happened and it is costing us big style now.
"We have conceded far too many late goals from sloppy play this season and with the experience we have that shouldn't be happening. My first target this season is 50 points and we should have been two points nearer to that."