Smith is out for the year following Tommy John surgery, but the Red Sox can try to get some value out of the trade from Elias, who will be promoted from Triple-A Pawtucket to make that start.
Elias has pitched just once for Boston this season, working in relief against the Astros on April 23.
At Pawtucket, Elias is 4-3 with a 3.54 ERA. He has given up two earned runs or fewer in four of his last five starts.
"You know, when we looked at Seattle and the matchups, we felt like a left-handed starter has proven to maybe negate some of their left-handed power," said Red Sox manager John Farrell. [Robinson] Cano, [Kyle] Seager, particularly. So this is as much matchup as the guys that are available to us. And Roenis' last four or five starts have been very good and much more consistent. [We] felt like it's time to take a look at him, particularly in this matchup."
Farrell picked Elias over Clay Buchholz, who was recently sent to the bullpen, and Pawtucket lefty Henry Owens, who struggled in an earlier stint with the Red Sox.
Joe Kelly, who was optioned to Pawtucket on June 1, is sidelined with a right groin injury.
• Left fielder Blake Swihart is out of the hard cast that had been covering his severely sprained left ankle. It is going to be a good while before he plays again.
"He's in a boot that still has him immobilized so he'll get some treatment but it's very minimized at this point," said Farrell. "There is no range of motion or work being done yet. He's still in the early stages of that immobilization."
• The news is better on backup catcher Ryan Hanigan, who has been on the disabled list since June 5 with a neck strain. Hanigan is eligible to be activated on June 20.
"Hanny feels better. He's got range of motion freely now from the left side of the neck that's restricting him. He will begin light baseball activity," Farrell said. "We haven't loaded that up to where he's going to get a true read on it. So, whether or not at the 15-day mark he's returned to us, that remains to be seen at this point."
• Brock Holt continues to take it slow in his recovery from a concussion that has sidelined him since May 19.
"With Brock, he continues to do baseball activity. When he was in Pittsburgh, he went through the impact testing, so the true symptoms of the concussion and the things that might interrupt decision making, that's cleared up," Farrell said. "But there is still some dizziness at times, the general fogginess that he feels, and certainly some headaches are there. So we're working toward getting closer to a rehab assignment, but no exact date for that yet."