We can create a partial regression plot on Number Of Workers, which shows the effect of adding Number Of Workers, while keeping the impact of % Downtime the same.
Notice the residual plot corresponding to the partial regression of Number Of Workers, and compare it to the residual plot for the simple regression of % Downtime. There is clearly a U-shaped pattern to the residuals. Compare this to the lack of a pattern in the residual plot for the simple regression of % Downtime. The U-shaped residual pattern indicates that a linear relationship is not the correct model for relating Number of Workers to the response variable.
In this case, zooming in on the partial regression of Number of Workers would reveal that the shape of the data more closely resembles an exponential relationship than a linear relationship.
