The 26-year-old, who became Barcelona's all-time leading scorer on Sunday, has struggled to produce his best form this season following a long-term thigh injury.
However, Sabella is of the opinion that the attacker will continue to develop in the years ahead.
"You always think that it will be hard for him to improve even further, yet he keeps on doing just that. Nobody knows what the limits of an extraordinary player like Messi are," Sabella told Goal.com.
Messi has scored 37 times in 83 games for his country.