With both Marner and Matthews together it leaves the Maple Leafs lineup thin as you go down the roster. The second line is fine with Nylander and Tavares. However, once you get to the third line the talent drops off immensely. Sliding Marner on that third line could give him better matchups and take pressure off of the top line.
Moving Mitch Marner could provide a major boost to the third line and overall depth of the Maple Leafs. With this strategy, the lines would look something like Matthews/Knies, Nylander/Tavares, and Robertson/Marner. These pairs would give two players on each line with high-end talent.