If you are considering defensive ratings, Monk comes in at 113.6 in the last twelve games, that is, since he began to start. Among rotation guards, that is behind Ellis (105.5) and Huerter (109.0). That is a little behind Sabonis (112) and one of the team's prime defenders, Keegan Murray (112.3). But it is also better than DeRozan (114.5) and Fox (115.43).
Huerter, Ellis, and Monk--in that order--all have similar defensive rebounding numbers per 36 minutes, between 4.2 and 4.0. Fox is next at 3.7 (minimum 100 minutes). No surprise, but Ellis has been the best on the team at steals per 36 minutes in the past 12 games, at 2.7, with Fox next at 1.7. Monk is way down on the list at .7, the same as Murray. Ellis also leads in blocks (1.4) although Isaac Jones has 2.2 per 36 minutes and has played nearly 100 minutes. Fox and Monk both have .7 blocks.
OK, net ratings among guards for the full season. Keon Ellis again leads the entire team at a phenomenal 9.1. Monk is next among guards at 1.4, Fox comes in at 0.0, and Huerter at -.07.
If you want to improve the team, start Keon Ellis, but there is no reason to trade Malik Monk. I say this as a dedicated fan of Mr. Monk. As Sabonis said before free agency last summer, "We can't lose him. We can't." He ought to know.