General manager Jerry Dipoto said Smyly saw two more doctors on Friday and Monday, and the consensus was the same as the original assessment by Mariners physician Ed Khalfayan, who diagnosed a strained flexor tendon and estimated at least a 6-8 week absence.
"Our timeline hasn't changed," Dipoto said. "It'll be six weeks till he begins throwing and eight until we can make a better judgment on when he will rejoin the club. We'll react conservatively to this. We want to make sure when we get Drew back, we'll get Drew back. We don't want to rush him back."
Smyly will join the Mariners in Seattle and begin rehabbing the elbow after about a week of complete rest following the PRP injection to speed the healing process.
Smyly, acquired from the Rays this offseason, was expected to open the year as Seattle's No 4 starter. His starting spot has been filled by Ariel Miranda, and now Marshall fills the 40-man roster opening.
Marshall, 26, was designated for assignment by the D-backs on March 28 after compiling a 3.38 ERA in 5 1/3 innings over six Cactus League outings. He's been optioned to the Minor Leagues and will report to Triple-A Tacoma.
Dipoto, who was with the D-backs when they drafted Marshall out of Kansas State in 2011, says the heavy sinkerballer has been victim of an extremely high batting average on balls in play (BABIP) the past two years.
"It's hard to believe that can be sustained, so we're just going to see if a change of scenery can help him out," Dipoto said. "He's healthy and no change in velocity, he's just not been getting the results."
Marshall pitched 15 games for Arizona last year with an 8.80 ERA and nine strikeouts in 15 1/3 innings. He was 1-1 with a 4.59 ERA in 33 relief outings for Triple-A Reno. In parts of three Major League seasons, he's 4-7 with a 4.50 ERA in 85 appearances.
Greg Johns has covered the Mariners since 1997, and for MLB.com since 2011. Follow him on Twitter @GregJohnsMLB and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.