If Iwakuma does get the $15.8 million qualifying offer, that doesn't guarantee he'll remain with the Mariners. All 34 Major Leaguers who received qualifying offers from 2012-14 turned them down and became free agents.
If Iwakuma rejects the offer, the Mariners still will attempt to sign him to a multiyear deal, though they'll be competing with MLB's other 29 teams at that point. If Iwakuma then signs with another club, the Mariners would receive an extra Draft pick as compensation.
But Dipoto said Karns' addition had nothing to do with any of that.
"I don't think we had a complete starting rotation to begin with," he said. "This addresses a need."
The Mariners will also return Felix Hernandez, Taijuan Walker, James Paxton, Roenis Elias, Vidal Nuno and Mike Montgomery as rotation candidates. Karns went 7-5 with a 3.67 ERA in 27 games (26 starts) last season before being shut down with a sore right forearm after reaching 147 innings.
Dipoto has no concerns with Karns' innings situation.
"He finished the season close to 150 and he's thrown more innings in the past," he said. "He's a big, durable guy. We've got the full medicals on him and are very comfortable with where he's at. The Rays would be the first to tell you if the season had continued and they'd been contenders, he'd have kept pitching.
"They're a particularly deep organization in terms of starting pitching and they took protective measures to shut him down. He appears 100 percent healthy and ready to roll. We have no concerns there."
Greg Johns is a reporter for MLB.com. 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.