The 31-year-old University of Florida product has spent his entire professional career with the Steelers since the team selected him in the second round of the 2011 NFL draft.
He's been limited to 12 appearances over the past two seasons because of injuries and a four-game suspension in 2017 for a violation of the league's policy on performance-enhancing substances.
Last week, it sounded like Pittsburgh general manager Kevin Colbert expected Gilbert to return as a starter in 2019 in comments made at the NFL Scouting Combine.
"We have the two starters—Al Villanueva and Marcus Gilbert," Colbert told reporters. "Matt Feiler did a great job stepping in when Marcus got hurt. Chuks Okorafor did a nice job when Matt got hurt and Marcus was out. So really, that's four guys who have played in legitimate NFL games."
Dale Lolley @dlolley_pgh
Sources tell me the Steelers had planned to keep Marcus Gilbert in 2019 but changed their minds in the past two days. The right tackle should find a solid market if he is released. We'll see what the trade market is. But a lot of teams could use a guy like him. #Steelers #dkps
Gilbert is entering the final year of his contract with a $6.6 million cap hit for 2019. The Steelers will save $4.9 million in cap space once the trade is finalized, per Spotrac.
The starting right tackle, who made 88 appearances (87 starts) across eight seasons, was graded as the NFL's 41st-best tackle by Pro Football Focus in 2018.
Feiler should get the first opportunity to fill the void for Pittsburgh, which is also dealing with the trade drama surrounding superstar wide receiver Antonio Brown.
Meanwhile, Gilbert will be tasked with helping upgrade the Cardinals line. The team ranked last in rushing yards per game (83.9) last season and allowed 52 sacks, tied for the fifth-most in the NFL.
Football Outsiders ranked the Cards' front five 25th in run blocking and 26th in pass protection.
A return to full health and top form by Gilbert would go a long way toward improving those rankings.