Starting Mazraoui at LB against Fulham makes sense for that specific game, but I’m against making it a long-term solution for two main reasons. First, Mazraoui is primarily a RB, not a LB. While he has occasionally played on the left for the Moroccan national team due to Hakimi's presence, he's only done so in 10 matches compared to 193 as a RB (according to Transfermarkt). He was brought in to compete with Dalot and serve as an upgrade over Wan-Bissaka, not to fill a LB role. Besides that, Mazraoui has been injury prone himself, not as frequently as Shaw, but still missed several games each season.
Second, relying on him as LB cover would leave us with just 2-3 reliable full-backs for two positions, which isn't ideal. Ideally, we’d target Ferdi, who can comfortably play on both sides (with 37 games as a RB and 68 as a LB). However, Ferdi might be out of our budget, especially with Ugarte being a higher priority. Alternatively, there are several decent LBs available for probably under €20 million, such as Locko, Dorgu, and Truffert.