Hard to say, but if i think at one single player importance for a club I managed it will be Julio Ricardo Cruz. I transferred him for my West Ham for half a million and he ended the season as top Premier League goalscorer and top Europe goalgetter.
Joe Mattock from leicter signed him for 2.5million and in my opinion with the right training is the best left back around because he is young , cheap and ENGLISH, after a season man city came in for him with a £12million bid i nogatiated £30million just to see what would happen and i could'nt belive my eyes when they offered it...
was a tough desition but turned it down because had 40million transfer funds already and didnt fell that i needed any one else in my team.....
and Bojan for free after 2 seasons is now worth over 20million and scores pritty much every game