I don't know how accurate team DefEff stats are, but the Tigers are 8th in the MLB in defense, according to BRef's DefEff rating. I recall this rating measures balls put into play (not HRs) that are turned into outs, which seems like a pretty decent way of evaluating the effectiveness of a team's defense. By this metric the Tigers are in the top 25%. I really believe the slow start is almost entirely due to the putrid slugging percentage, which is second to last in MLB. The Tigers are dead last in MLB in HRs. Every hitter in the lineup with a track record is off his HR pace, even when one factors in the reduction in HRs so far this year.