Diamond was designated for assignment Tuesday, Gregor Chisholm of MLB.com reports.
Diamond had just joined the major league roster Monday, but he struggled in his lone appearance with the big league club to the tune of two walks and three runs over just one inning, so he'll head back to the minors. A corresponding roster move isn't expected to be announced until Wednesday, but Diamond figures to be a candidate to get promoted again later in the season should Toronto find themselves in need of pitching depth.