Matthew proposed to Mary (Michelle Dockery) in the show's Christmas special and she accepted him.
However, speaking at a screening of the episode earlier this month, Stevens explained that he is not sure that their relationship will be completely smooth.
"[What happens to them] really lies with [writer] Julian [Fellowes]," he said.
"Of course, we don't see them married [in the Christmas special]. There's got to be a wedding. They've got to get to the altar. Who knows? We see them getting engaged and that's resolution enough for this episode."
Stevens has previously joked that keeping the plot twists a secret is "part of the fun" of starring in Downton Abbey.
Filming for the third series of the show, which will be set in the 1920s, is due to begin in February.