Rashford, 27, has scored three goals and provided five assists in 10 games for the Spanish side since joining from Manchester United. While Tuchel praised Rashford's ability, he challenged the player to improve his consistency.
"He can be one of the best in the world," Tuchel said. "But the numbers don't reach the potential. It's as easy as that."
Rashford has returned to the England squad after falling out of favour in 2024. He featured off the bench in a 3–0 win over Wales and could appear again on Tuesday when England face Latvia, needing a win to qualify for the 2026 World Cup.
Tuchel said the issue isn't talent, but performance.
"He has to constantly prove himself - whether starting or coming off the bench," said the Three Lions boss. "Otherwise he may look back in 10 years with regret."
Rashford previously suggested his dip in form was linked to instability at United. Barca manager Hansi Flick has praised his impact, and the club has a 30 million pound option to sign him permanently in 2026.
Tuchel also confirmed Jude Bellingham was left out of the current squad due to injury recovery, but said the Real Madrid midfielder would rejoin the team's leadership group upon his return.
Arsenal captain Martin Odegaard is expected to remain out of action for another six weeks as he recovers from a knee injury sustained earlier this month, according to reports.
Barcelona's injury woes have worsened with the news that star striker Robert Lewandowski has suffered a muscle tear in his left thigh and will be out of action for at least six weeks.
Jurgen Klopp has come to the defence of Florian Wirtz, calling him a "once-in-a-century talent" despite the midfielder's slow start at Liverpool, and urging patience as the young star finds his footing.
Frenkie de Jong has put an end to months of speculation by agreeing to a new contract with Barcelona, set to extend his stay at the club until 2029.
