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At 21 some players have a purple patch and go on to finish their career in League two, China or... Rangers.
Just because someone is playing at 5/10 every game at 21 doesn't mean they're destined to play a 7/10 every game aged 26.
Youri Tielemans has already shown definitive progress and is 100% better than Longstaff AT THIS TIME. Doesn't mean he'll necessarily be better in 5 years obviously.
First of all, you are comparing players who play in different positions. Tielemans is more of a box to box attacking midfielder whereas Longstaff is a more defensive minded midfielder. Secondly, how can you say Tielemans has shown definitive progress when he flopped at Monaco?