Agree with pretty much all DB's sentiments - an undeserved scoreline but with a team this new and young, there's going to be a few like this, and i'm not looking forward to the Super Cup. Once Rudiger and Kante are back we should be a lot more solid defensively though - exhibit A being Kante's incredible last ditch tackle to prevent Rashford's hattrick.
Interesting question for me is how the first choice midfield looks at full-strength, if we assume Pulisic/Willian/CHO/Pedro take the two wide positions and Kante is locked in for one of the deeper roles. There's a three-way fight for that #10 role between Barkley, Mount and RLC, and then a two-way between Kovacic and Jorginho for the other deep role (which RLC could also potentially take). I'm not sure yet who deserves those spots, but Barkley and Jorginho are probably in the lead right now.
As DB stole most of my thoughts on the game, i'll instead offer some unsolicited player rankings:
Kepa: 7/10 - Couldn't do anything about any of the four goals: penalty, tap-in, one-on-one, deflected shot. Distribution was decent and made a few tidy saves, one of the few calming influences at the back.
Dave: 4/10 - At fault for Martial's goal although a tough situation against a substantially bigger attacker. Given he doesn't offer a lot going forward he needs to be at his dominant peak defensively, and i'm a little concerned he has lost a step of pace and is no longer quite at it. Also questionable positioning/communication for the third goal. Few strikeforces will have as much pace as Martial-Rashford though, so better days ahead.
Christensen: 5/10 - More composed on the ball than he was at times last season, but lost Rashford for the third. Otherwise made a few key tackles and blocks - I think he's a step down from Luiz but is now comfortably our best CB partner for Rudiger.
Zouma: 2/10 - Shocking first half, although not really responsible for the second-half goals. I think the pace really troubled him, and that he'll be a more effective CB against more physical, less quick strikeforces. I still believe in him long-term as a serviceable squad player, but don't think he'll ever be comfortable enough on the ball to be a nailed on starter.
Emerson: 6/10 - The best defender today, got unlucky with the deflection on the last goal, but perhaps could have done more in the build-up to the second. Solid striker of the ball and far more handy going forward than Dave, which is going to mean a lot of our attacking play this season runs down his flank. Unlucky to hit the bar.
Jorginho: 6/10 - For all the talk last season of needing to play a 4-3-3 with Kante more advanced to fit in Jorginho, he actually looked more comfortable next to Kovacic, which was fairly amusing. With the inexperience around him he seemed to take a leadership role in the middle of the park, and think he's going to be key for us this year as a playmaker - his role will be very different once Kante is next to him I imagine.
Kovacic: 5/10 - Typical decent at everything while not standing out for anything in particular performance, think he'll find starting roles hard to come by once RLC and Kante are fully fit, which makes the big fee we paid for him appear like it was exclusively down to the transfer ban.
Barkley: 7/10 - Thought he was our most dangerous player and not sure why Frank brought him off rather than Mount or Pedro (or why he got shunted to the wing). Like Jorginho, I think our fate this season is going to fall in large part on his shoulders. I think he's got the physical and technical tools needed to be our starting attacking mid, just needs to pick up the confidence.
Mount: 6/10 - Very surprised to see him start, but he looked very comfortable. Faded once we went behind, I think he'll be better suited to coming off the bench this season while he finds his feet, but we can also feel comfortable about starting him following this performance. Also potentially our best long-distance threat.
Pedro: 4/10 - Think he was our least effective attacking player although because of his effort on both ends, he'll always at least offer a 4/10. Made a couple of wrong decisions when he could have played in Tammy, and think his role is under peril once CHO returns.
Tammy: 5/10 - Decent performance and unlucky to hit the post, he's not a top striker yet but on today's performance he has enough in his bag to start a lot of games this season at least. I still think we'll need to spend big on a striker next summer, and that Tammy will slot into a backup role going forward. Questionable whether he's already better than Giroud - against defences which sit deep I think Giroud's physicality and passing will make him more valuable.
Subs: Kante 7/10, we missed him so dearly and he stopped a near-certain goal with an incredible recovery tackle; Pulisic/Giroud 5/10 - Unfortunate they both came into the game while the defence self-imploded and game suddenly became lost.
Frank: 6/10 - Tactically he appeared to get it right, before being undone by personal error and some unlucky finishing. Substitutions were positive. At Derby he was very attack-minded, but like Sarri and others before him, he may need to start treating these tougher games a bit more pragmatically in-future. I think he would have learned a lot about his personnel today, and I hope the club will be patient with him.