To make room on the roster, the 49ers have waived cornerback Dontae Johnson.
The move comes as starting tight end George Kittle deals with knee and ankle injuries suffered during the 49ers' Week 9 victory over the Arizona Cardinals. He is not expected to play against the Seahawks, and Celek's activation is a good indication that he won't.
Celek started practicing again with his teammates last week.
"Yeah, he had a real good week of practice," head coach Kyle Shanahan said on Saturday. "We only have one day of pads, though, so there's only so much stuff like that. He obviously moves around really good right now.
"He's healthy, he's played a lot of football in his career, but you'd definitely rather give someone a few more weeks to get acclimated back into the stuff, so we'll see if we have to [activate him] or not."
Shanahan discussed Celek's injury back in June, saying, "It's a serious surgery he did, dealing with the back. Some stuff that Trent (Taylor) went through last year, and some stuff that Celek went through before he got here. I think five years ago he had a surgery like this on his back."