His revelation casts doubt on the theory that all 298 passengers on board died instantly when the plane was hit by a missile over Ukraine July 17, BBC reported.
An initial report last month said flight MH17 broke up in mid-air after being hit by "objects" that "pierced the plane at high velocity".
He mentioned the oxygen mask during an interview Wednesday night aired on Dutch TV in which he was taken to task about an emotive speech he gave to the UN four days after flight MH17 was downed.
"But do you know that someone was found with an oxygen mask on their mouth -- and so they had the time to put it on?" Timmermans said during the interview when asked if he had created an image that had not really taken place.
He went on to say that nothing could be ruled out about the 298 victims' final moments.
The official inquiry into the MH17 disaster has not made any mention of an oxygen mask being found on one of the victims.
But several experts have concluded that the plane would have disintegrated too quickly for the passengers to have known anything about it.
The plane had been flying from Amsterdam to Kuala Lumpur when it crashed in eastern Ukraine July 17.
Although investigators were unable to visit the crash site because of fighting in the area, their initial report pieced together photographic evidence of the wreckage as well as cockpit and air traffic control data.
They said it pointed to "an in-flight break up" and added there was "no evidence of technical or human error".
The plane was hit while flying at 33,000 ft and debris was found over a wide area of eastern Ukrainian territory held by pro-Russian rebels.
The plane had 298 people on board, of whom 196 were Dutch. So far, 262 of the victims have been identified.