Tackle Garry Gilliam, a restricted free agent with the Seahawks, signed an offer sheet for a one-year deal with the 49ers on Monday.
Gilliam, 26, visited the 49ers last week. His signing of an offer sheet first was reported by ESPN, which said the deal was for $2.2 million overall and $1.4 million guaranteed.
That means the Seahawks have five days to match the contract or they will lose him to their division rival. Because Gilliam (6-5, 315) went undrafted in the 2014, the 49ers would not have to compensate the Seahawks with a draft pick.
The 49ers have adequate depth on the interior of their offensive line after trading for Ravens center Jeremy Zuttah last month. Zuttah also has played left guard in Baltimore.
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But their depth behind Joe Staley and Trent Brown at tackle is limited to John Theus, who started one game last season.
Gilliam has started 30 games over the last three seasons, most of them at right tackle. He began as a tight end at Penn State before converting to tackle and only spent one season at the position in college.
The Seahawks, who made several upgrades to their tenuous offensive line in the offseason, tendered Gilliam at the lowest level possible.
The Seahawks at one point had high hopes for him, earmarking him as their left tackle for 2016. But when he struggled in the preseason, he was moved back to right tackle and then eventually was replaced as the starter there.
When his replacement, Bradley Sowell, ended up being even worse, Gilliam was re-inserted for the final two regular season games and for the playoffs.