Fleming found himself in acres of space against ten men Chelsea, allowing him to nod home a dramatic equaliser for the Premier League strugglers.
Chelsea have now dropped 19 points from winning positions in the Premier League, 17 of which have been at Stamford Bridge.
Speaking to reporters after the game, a clearly frustrated Rosenior blamed a mystery player for not marking their man.
“An assignment was missed,” Rosenior said.
“An assignment, a marking assignment, wasn’t done. Flemming, we know, is their best header of the ball. And there was a player who I won’t (name).
“I’m not here to throw players under the bus. I will always protect my players, and I will deal with it in the week. There was a player we assigned that duty (and he) marked the wrong player.
“We knew their biggest threat with (James) Ward-Prowse on the pitch was set plays. We went as big as we possibly could because that was the only way they could score with 10 men. We still don’t see it out.
“I think the frustration is more (with) the last two home games. From being in winning positions and being in control of the game to not win the two games, that’s the biggest frustration.”