McClaren saw his Rams edge past relegation-threatened Rovers, thanks to goals from George Thorne and Chris Martin in each half, to keep their slim hopes of stealing second place off Burnley alive.
But with an eight-point gap to make up on the Clarets in three games, McClaren is already preparing for the play-offs.
"It's too little, too late," he said. "They (Burnley) are going to go up. I can't see them falling over now. Getting over the line is hard, but I can't see it."
Derby's place in the play-offs had already been assured before kick-off and a lethargic first-half display followed.
Thorne's stunning 25-yarder on the cusp of half-time was barely deserved before an improved second-half performance was capped by Martin's 19th league goal of the season.
"It's about performance and we were bitterly disappointed with the first-half performance," McClaren said.
"The second half was better but we can't play like that in the first half, better teams will kill us. We have to do better."
McClaren will hope for more from his side in their remaining three games before the play-offs, where he suspects his side will be underdogs.
The Rams sit third in the table but McClaren is wary that they could be joined by QPR, Reading and Wigan - the three teams relegated from the Premier League last season.
The former England boss may have been attempting to employ some early kidology ahead of those cut-throat meetings next month, but certainly few would have tipped his side to go up before he took charge.
"We'll just concentrate and focus on the play-offs now," he said. "Looking at it you are at least going to get QPR in there and Wigan.
"The play-offs are a cup. You see over the years favourites go out.
"Usually experience counts and with that QPR, Wigan and Reading - they've got the squads."
Defeat for Doncaster deepened their relegation fears, with a crucial trip to fellow strugglers Millwall now looming on Monday.
Defeat was Rovers' fourth on the spin and they will head to London with just a three-point gap over Millwall, who sit just inside the bottom three.
"We have to go down there and get a result," boss Paul Dickov said. "It's imperative that we do.
"We were everyone's favourites to go down at the start of the season and have been all season.
"But the great thing is we're in a position where it is in our own hands.
"It will be a massive, massive achievement if we stay up this year."
Dickov expects striker Chris Brown will be available against Millwall, despite coming off with cramp in his hamstring, while Lucas Neill and Richie Wellens are also in line for recalls after injury.
"Hopefully we might have one or two back," he said. "Browny's hamstring has just cramped up a little bit. As soon as that happened it was pointless keeping him on, with Monday in mind."