What makes you think that? They've already gone big on Cunha, almost 3 weeks since Mbeumo approach surfaced and no second bid reported, I think United are going to wait and see if they can shift the likes of Sancho and Garnacho before they dedicate funds to replacing them. If we are given time early in the window where United can't act but we can, I think we can steal a jump on them and bring Mbeumo here before they're in a position to compete for his signature. If Spurs are offering you £180k with bonuses right now, and United are telling you "if we can sell X player to Y club we will bring you in after", are you hanging on to see if anyone is stupid enough to take Rashford's £350k/week wages off their hands or are you taking the offer that immediately quadruples your salary?
The latest players linked to Utd are Gyokeres and Ekitike, both #9s, I wouldn't at all be surprised if Cunha/Mbeumo was an either/or for United and they've pulled out of the Mbeumo deal having secured their preferred target also. Amorim largely seems to play 3-4-2-1 at Utd and Bruno is a shoe-in for one of the AM spots, so you fancy Cunha will take up the other with a new #9 going up front.