No signing is terrible at the time, but Cancelo was a luxury signing, and in hindsight has shown to be a poor signing. A few decent games at the back end of the season with the pressure off is not a sign of greatness.
We simply don't know if Mendy was a good player or not, he was injured immediately after signing but at the end of the day he could have been very overrated at Monaco
And neither is it a sign of a poor player; let's just hope that Cancelo proves that he is the class player he looked like he was becoming.