Kyle Shanahan and John Lynch explain the trade of Daniel Kilgore to the Dolphins

San Francisco 49ers general manager John Lynch and head coach Kyle Shanahan discussed Thursday's surprise trade of center Daniel Kilgore to the Miami Dolphins just a month after re-signing him to a three-year extension worth $11.78 million with $7.05 million guaranteed.

The trade ended speculation on who will start at center for the 49ers in 2018. San Francisco signed free agent center Weston Richburg to a five-year deal worth up to $47.5 million on Wednesday, and some believed the two would compete for the starting job with the loser possibly moving to guard.

During a 30-minute press conference held on Thursday afternoon to introduce the 49ers' newest free agent additions, Shanahan explained the decision to trade Kilgore.

"Mainly because we see Dan as a center," the head coach told reporters. "He does also. I don't think that would be fair to put him at guard. This situation with Dan is one of the hardest things I've gone through as a coach. I've got the utmost respect for Dan. I think he's a good player. He's even a better person. That's why this was so tough.

"But when we had an opportunity to add a center of Weston's caliber, and to improve our interior like that, and when we go into free agency, and we watch all the O-lineman, and how can we improve this the most, I think we all personally felt that [Weston] was the top O-lineman that we saw out of all the inside guys.

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"(We) didn't know if we could add him or not, and when he expressed that he wanted to be here and it worked out, that's an obligation that we kind of have to this organization when it comes to age and things like that, and what you're getting. That's a tough decision we had to make.

"By no means was it easy to let go of Dan or to give him another opportunity somewhere. I wish we could sit him here, hold him, and use him if we needed to but our team has gotten a lot better today.

"We wanted to do very right for Dan and his wife, and Miami is something they were interested in too, and something we were communicating with him and his agents on."

Lynch said they knew there was going to be a lot of competition for Richburg so there was no guarantee the 49ers would be able to sign him.

"We had to compete vigorously for him," Lynch said. "There were a lot of suitors ... We liked Dan Kilgore a lot. We like what he is as a player. We love what he was for us in the locker room as part of setting the tone."

The 49ers swapped seventh-round picks with the Dolphins during the exchange for Kilgore. That means the 49ers moved up just four spots in the round.

"It was more about finding the right place for Danny and a great opportunity for him because of what he's given us. Fortunately, that presented itself with Miami, and I think he's in a good spot there ... They've got great plans for him, and that makes us happy."