You may not be able to stash him in an "injured" spot anymore now that he's been removed from the DL and optioned to the minors, but Taijuan Walker is still worth stashing in most Fantasy leagues. In fact, I'd say he's the most stashable prospect in baseball right now.
Let's not forgot the debate over "best pitching prospect" coming into the season came down to him and Archie Bradley, and of the two, he's still the closer to breaking through even with this demotion. Bradley is, of course, still working his way back from an elbow injury while Walker already has five starts under his belt.
Granted, not all of those starts were good. His last three -- the only three at Triple-A, in fact -- are probably what convinced the Mariners to hold off on him now, but considering he was expected to claim a rotation spot before hurting his shoulder this spring, it won't take much for him to win them over. Maybe just one great start with only Erasmo Ramirez standing in his way.
He looked plenty prepared for the role in three starts last September, making him less of a mystery than Andrew Heaney. With hitters like Gregory Polanco, Oscar Taveras and Jon Singleton already up, there really isn't prospect more deserving of a roster spot. If any minor-leaguer is worth stashing in your league, it's Walker.