I would imagine he'll be back on the 1st line within a week or so of him coming back from injury.
That's where he was to start the season and the line was tearing it up. I think the Sharks would prefer putting Marleau on the second line to spread things out a bit so Seto should take his spot.
Just my thoughts....I'm hoping he gets back on the 1st line too, he's on my fantasy team.