Rovers were being bossed out of the game by an intense Cambridge outfit who led at the break.
But within the first 12 minutes of the second half, Ferguson’s side led 3-1 and ran out comfortable winners at the Abbey Stadium.
“I just said to the lads at half-time if tomorrow night you’re watching the cup draw and Cambridge get Manchester United or Chelsea, you’re going to be gutted,” Ferguson said. “I said you’re going to have to do something about it.
“And credit to my players again, that’s another game where they’ve come back from behind.
“We didn’t win at Coventry but we came back really well and we came back to beat Cambridge.
“In some games, especially cup games, you just need to get into the next round.
“It wasn’t an exceptional performance but in some parts it was very good.”
Harry Forrester was absent from Rovers’ matchday squad and Ferguson revealed it was simply due to not being selected.
He said: “I picked my squad accordingly and what I felt I might need on the bench.
“There was no other reason behind that.”
Rovers will play a behind-closed-doors match with Rotherham United tomorrow with those players not used at Cambridge set for game time.
Curtis Main and Richard Chaplow will both return to training on Monday.
Nathan Tyson suffered a back injury at Cambridge which forced him off at half time while Dany N’Guessan will be out for another week due to a hamstring issue.