Sanchez endured a miserable Premier League season, but he scored his second Copa America goal in as many games in a 2-1 win over Ecuador.
The attacker also played through an ankle injury, scoring a 51st-minute winner to move onto two Copa goals, which is more than the one he managed in 20 Premier League games in 2018-2019.
Rueda feels Sanchez's struggles domestically are a reflection of United's battles, but he also believes the 30-year-old is more loved with the national team.
"He has the motivation, affection and some relationships that he has in the national team and maybe he didn't have them in Manchester," Rueda said after Chile moved into the quarter-finals.
"He arrived at a difficult, inopportune moment, perhaps in the most unbalanced United in recent years."
Chile will advance from Group C, but must avoid defeat against Uruguay in Rio de Janeiro on Tuesday (AEST) to secure top spot.