He might be, but could equally play in a 2 with Sterling.
Also even when we play 433 he's not really on his own because Sterling and Sané have been playing so far inside.
Mendy's injury is more likely to effect him IMO than Sergio's - no Mendy will probably force Sané to stay out wide on the left to provide width and might leave him a bit isolated.
Hey, I already said that at the bottom of my post ;P
I know what you mean with Mendy, but we didn't have him last season and the Sane Sterling Jesus trio seemed to work