[6][7][8] On 22 February 1903, Košek made his debut as an 18-year-old, in the red and white colours of Slavia Prague, in a 9–1 win vs CAFC Vinohrady.

On 1 April 1903, Slavia Prague acquired the pupil of the Academic Gymnasium Jan Košek and publicly admitted that the transfer took place (for the first time in our football history) for money.

During this time, Košek was still studying at the academic gymnasium and had to hide from the school authorities in newspaper reports and in photographs.

By February 1904, an Austrian newspaper stated, "Košek has developed in such a way, in recent games, that he should probably counted amongst the best strikers on the continent", despite being only 20 years old.

[23] Jan was highly coveted by Vienna Cricket and Football Club and by Rangers 1906 was a great year for him and Slavia Prague.

[48] In a game against Aberdeen, the score was 2–2 and in the 82nd minute, Košek took the ball on the half turn and rifled it into the net from 30 meters.

Rudolf Krummer, Richard Veselý and Hajný played in the defence, Emmanuel Benda, Majzl, G. Macoun and Holý were in the middle and Medek the attacker.

However, the SK Slavia committee is sure of his services, as the retired footballer has the best credentials to become a capable club official based on his rich experience.

[55] 1914 sparked a change of decision from Košek, as he agreed to play several games for Slavia Prague, due to lack of players.

In his final recorded game for Slavia's first-team, Kosek scored 2 goals and registered an assist, in a 7-0 victory over First Vienna.

Upon announcing the squads to face each other, in the Bohemia–Brussels match, the author of Le Soir said this about Košek: "Jan Kosek, captain of Slavia, the most famous left-forward on the continent.

Taking advantage of the fact that the ball was wet, heavy and slippery, he often hit from a distance, and our poor goalkeeper did not have the strength to stop such shots and the result 10–3 to Slavia Prague.

Another author, this time of Sport-Vílág stated: "The main strength of the Czech team lies within the attack, in which Sharp (Kosek), the continent's best player, plays.

"[71][72] Mr. Jenšík and Mr. Macků write in the Chronicle of Czech Football: "Jindrich Baumruk just lifts his leg, he magically catches the ball on the tip of his foot and turns as if on a spinning wheel and already kicks it down the line.

Ferdinand Scheinost, journalist and writer, considered Jan Košek the greatest football personality he had seen and experienced.

[74] An author of, Večerní České slovo, wrote; "Today we buried a friend, a rare athlete, the type we wish we could generalize in our sport.

Slavia had more of them in the history of their football teams; I randomly remember only Baumruk, Setzer-Bloomer, Jul.Kosátku, Karel Kryž, Kindle, Maizlo, but Košek stood above all the exuberance.

He fully understood that the purpose of the match was a goal, and none of our players heard the cheers of thousands as many times as the late Košek, when he scored a lucky shot.

When an opponent explained to him that he had no sense of combination, Baumruk always took him up on it and declared: "Košek was the ideal of the youth, in the stands, who would watch with devouring with eyes.

The author wrote about Belfast Celtic's trip to Bohemia in 1912, "Then there was the prolific inside-left by the name of "Kojak" who was reputed to have scored 800 goals in just 5 seasons".

With Kosek, disappears a player whose class had never been reached so far in Czechoslovakia, even by Káďa, Pilát, Kolenatý and Plánička, the stars of today.

[87] G. O. Smith mentions: "Hejda was good in goal, Jindrich Baumruk on the wing literally ran past our defender, but the best of all is Košek.

Yesterday, we are the ones who came to show him his last honor, it was the best proof of Košek's popularity despite the fact that this perfect athlete stopped exciting the masses with his game a some time ago.

He would like the mass football youth to have his love and passion for the sport, to be unafraid of fighting, to always be the brave heart of Košek in the game, to be manly, unyielding, tough and honest in training and in matches.

the memories of his glory as a player and as an athlete, may those who knew him awaken in today's football generation a living desire for such perfection as was achieved by this son of poor parents, who, with his skill, did a lot of work for his little one nation.

[25] In the Večerní České slovo, an author states his best combined team in Czech history and best 11 players in Czech history: "If only we could build, the strongest line-up of Czechoslovakia would look like this: Plánička, Pospíšil, Hojer A, Kolenatý, Káďa, Pemer, Baumruk, Svoboda, Pilát, Košek, Mazal.

"[89] The memories of this player, the most feared attacker, the most vivid we have ever had, a shooter who in the years of his highest performance was unmatched by anyone on the mainland and who has not found a follower in our country to this day.

If anyone was particularly responsible for the excellent reputation of our football many years before the war, it was certainly Jan Košek, this phenomenal forward of Slavia.

[90] On 4 January 1928, an author of České slovo wrote; "Yesterday afternoon, a large number of athletes and friends of sports, especially the older generation, gathered to pay their respects to a man who, as a football player and athlete, contributed in an excellent way to the popularity of our sport at home and its good conduct abroad.

[96] Every year, the Slavia Prague delegation commemorated the anniversary of the death of Jan Košek by placing wreaths at his urn at the Vinohrady Cemetery.