Ospreys backs coach Gruff Rees said they had been beaten by a more experienced team after losing 20-9 at home to Northampton.
The result at Liberty Stadium ended Ospreys' chance of qualifying for the Champions Cup quarter-finals.
"The most efficient side won," Rees said.
"And even though there's glimpses in our game which are really positive, and aspects of that first half we really enjoyed, the scoreline doesn't lie."
Ospreys were made to pay for mistakes, as Calum Clark capitalised on Tom Wood's charge-down of a Rhys Webb kick, while Wales wing George North picked off Justin Tipuric's dropped ball to score against his countrymen.
"They squeeze you into certain areas and two mistakes, they get 14 points," Rees added.
"That's difficult to take but that's the reality and we played like what we are really, the youngest squad in Europe and it just shows through in our rawness sometimes."
Ospreys complete their European campaign next week in the dead rubber against Treviso, while Northampton host Racing Metro in a game that will decide who tops Pool Five.