Albion went to Wearside in the knowledge only an outlandish series of results could relegate them to the Championship, and Sunderland won 2-0 to ensure their own survival.
Pepe Mel's side are still not officially safe, although it would take defeat at Stoke and a Norwich win against Arsenal - with a 17-goal swing - to send them down on Sunday.
Despite that, goalkeeper Foster says Albion owe it to their supporters to finish the season on a positive note.
"Sunderland probably just wanted it a bit more than us," he told the club website. "They were pretty good in the first half, but our standards were way below what we're used to putting out and it's not really good enough.
"Maybe it was something to do with the big relief that we're pretty much safe now. There has got to be some stupid turnaround in goal difference for us to be relegated so there was a big relief for the lads, but still we had a job to do and we didn't do it.
"We're really disappointed for the fans, who did a really good job and were really loud and noisy, but we didn't put a performance on. It was a good following and we should have given them more than they got.
"We are pretty much safe now but we need to finish on a high against Stoke for the fans. Then we just need to look towards next season.
"It was almost like we were just going through the motions and there was no kind of fight there. So we're disappointed, we understand that the fans are disappointed. We let them down a bit."