I don't own Shields, but the matchups aren't favorable for TB
upcoming opponent x # of series
BOS x 5
TB is also scheduled to face contenders like SEA and LAA + TOR.
Shields likely won't get a start in all the series above, but I would be hesitant to start him against BOS, CLE, DET, and NYY. It looked like batting practice last couple times he faced the pinstripes and even many of his outs were line drives. On the plus side, TB does face BAL 4 times and I would think Shields might dominate in those starts even if Tejada is back.
Read what you will into TB's schedule. If I owned him, I'd be inclined to sell ASAP