My initial thoughts on the USO19 final: court positioning, power and approaches were obviously relevant. But Med's biggest issue was his shot placement on his big target strategy.
His big target point rally construction was aimed at the wrong side. Nadal is always looking to hit the FH but he is most dangerous on the BH side where he can run around it and hit the FH inside out or inside in. Med played most of his rally balls to that centre-BH side which is exactly where Nadal wants to be. Even if you get enough on it to stop him running around the FH, Nadal has built his CC BH to be nearly as deadly as his inside out FH. Med was behind in a lot of rallies from that shot on. Med's deep positioning that he has been criticised a lot for actually helped him stay in those points.
The few points where Med's radar drifted and he accidentally rallied balls to Nadal's centre-FH side, Nadal didn't do much with them. The DTL FH from the centre-FH side is a difficult shot and one Nadal doesn't play much or with much threat to it. He also can't put much on the CC FH from the centre of the court, so didn't damage Med much from there. Med turned a lot of points his way starting with these rally balls to centre-FH. This was evident throughout the match but becomes very noticeable here from the 3rd onwards when he started getting more aggressive on the weak replies from Nadal's centre-FH side.
His decisions on counter attack bring into consideration all the court positioning, power, winners and approaches issues that have already been covered. His counterattacking game definitely needs improvement across the board there but not as much as you think. When he lets his attacking instincts take over and he's close to the baseline between 3 and 10 shots into the rally, he does a lot of damage to Nadal in a lot of ways. The main issue is with putting himself in too many defensive rallies.
The slices. They're not troubling Nadal, at all. Too soft, not enough pace, not enough slice and too much side spin which makes it readable in the backswing. Again, always to the wrong side. Nadal actually likes slices to his BH that he can reply with ludicrous BH DTL moonballs or run around and get underneath them with the inside out or inside in FH.
He and his coach clearly study the game and the psychology required and understand that adjustments need to be made. Now he sees he has a shot at this level, there is lot to work on and some choices about what aspects of his game and strategy to prioritise. Maybe they look at his conditioning first, or power, or spinrates or approaches: slice, angle groundstrokes and volleys. It'll be interesting to see how he fares after time to rest and regroup for WTF and AO20.