It was Sale's fourth consecutive loss and, even at this stage of the season, severely dented their chances of making the top six.
Despite fighting back from 26-9 down and getting to within seven points, the Sharks could not force a result, much to the unhappiness of Redpath.
"There weren't any plus marks really as most of our game wasn't up to speed," he said.
"You can't hide behind that performance tonight because, as a group, it wasn't good enough.
"It let a lot of people down that came along tonight. We need to find out why, but there's not many answers coming out at the moment.
"We really need to look hard in the mirror and see why that happened tonight as that wasn't acceptable at all.
"I don't want to take any credit away from London Welsh. They came up here, played a simple game, got in front and stuck at it.
"I can understand the frustration and accept it, but we have to take that responsibility when the fans booed us off the pitch and that is something we obviously never want to happen again.
"I apologise to every Sale supporter. They pay good money to watch and I will be doing my utmost to rectify that."
London Welsh secured their second victory in five days to the surprise of head coach Lyn Jones.
Gordon Ross kicked 19 points while Jon Mills and Ed Jackson went over from close range to defy the critics who wrote them off at the start of the season.
Jones said: "Compared to the first two weeks, are we surprised? Yes, very much so. You've got to be, haven't you?
"But the more you play like this, the less surprised you get. We got caught with our pants well down against Leicester and Quins, but we've learnt and we've come back.
"If you had asked me two weeks ago if we would have settled for a losing bonus-point at Sale, I would have bitten your hand off.
"Today's a good day for us, as last week was, but you know and I know that next week could be a bad day, so we've just got to keep on improving.
"We spoke long and hard in the week about our start, conceding 14 points and playing buffet rugby.
"Today we started well, we started confidently, we looked in good shape and took 13 points in the first 10 minutes which was the difference at the end of the game."
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