Assuming an average hitter in place of Bell gives us 78 RBI instead. That means Bell produced an extra 18 RBI compared to an average batter. Of course, if Bell doesn’t drive in those runners and doesn’t make the 3rd out, the next batter would have had a chance for the RBI, and we can assume they drive in 15%, which negates roughly 3 of those extra RBI. That means Bell drove in 15 more runs than an average batter in his situation.
He is also really slow, so he is scoring runs at a less than average clip. He scores a -3 Rbaser, Subtract those from the total.
That means his -1.4 WAA is pessimistic when you consider the actual luck he had driving in runners. Add back those extra 12 runs and he’s a -0.2 WAA player. A C- might be a little harsh. I’m willing to give him a solid C. The Twins probably finish 75-87 without him.