Former Portugual international goalkeeper Quim says Porto No99 Diogo Costa has the potential to become one the country's greats.

Costa is being linked with Manchester United this week.

Quim told O Jogo: "It's not just about penalties. It's a matter of study and skill, but he's worth much more. In modern football, he's the best there can be. Of course, there's always room for improvement in every aspect, but I don't see any weaknesses.

"It gives the defenders peace of mind, it's as if nothing happened. And, as you've seen, it's an asset in the depth he offers."

Keeping the young goalkeeper in the long term "will not be easy" and Quim projects a future at the highest level for Diogo, "in whatever club".

"Maintaining this level... He will have no difficulty in imposing himself, because he will always cost many millions. The best clubs will be on the lookout. I have no doubt, he will be one of the best in the world."