Rovers were comfortably beaten 3-0 by Oxford at the Kassam Stadium as James Henry’s brace and Cameron Brannagan’s strike took the hosts out of sight.
The game marked the welcome return off the bench for striker Ennis after recovering from a hamstring injury.
But he took no personal pleasure from the outing and felt he should have contributed more himself.
He said: “As a striker, you want to come on and affect the game and get a goal.
“I didn’t do that so I’m not happy with how I played.
“I have a standard of how I want to play and I don’t want to drop that even if I’m coming back from injury.”
While unhappy with how Ennis measured up to his own standards, there can be no doubts that Rovers dropped below their own typical benchmark, too.
They struggled to find the cohesion and understanding they have shown for so much of this season as they faced a strong and intelligent press from Oxford.
It saw them invite pressure as they attempted to play out from the back and also struggle to create their own opportunities, ending the game without a single effort on target.
They had looked to have ridden out a tough first half only for Henry to thrash in a shot from 20 yards via a deflection off Donervon Daniels in stoppage time.
Two minutes into the second period and they had a mountain to climb when Brannagan surged into the area untracked and slotted home from Chris Cadden’s cut-back.
The icing was put on the cake for Oxford 13 minutes from time when Henry drilled in from the penalty spot after Tom Anderson was harshly adjudged to have handled.
Ennis said: “It was a frustrating game obviously but we still managed to get in the right positions, It’s just about getting the end product right.
“I just felt that on any other day we’d have taken our chances. We just need to keep improving.
“I felt at 2-0 we were still in the game and creating cances which was a positive.”
Oxford United: Eastwood; Cadden, Dickie, Mousinho, Ruffles; Gorrin; Sykes (Baptiste 68), Henry, Brannagan, Fosu (Hall 79); Mackie (Agyei 87). Unused substitutes: Stevens, Moore, Long.
Doncaster Rovers: Dieng; Halliday, Anderson, Daniels, James; Sheaf, Whiteman; Sadlier, Coppinger, Taylor; Thomas (Ennis 64). Unused substitutes: Lawlor, Wright, Greaves, Gomes, Kiwomya, May.
Referee: S Stockbridge (Tyne & Wear).