Roberto Martinez, the head coach of Portugal, has spoken highly of Sporting striker, Viktor Gyokeres, calling the Swedish player one of the best signings in Europe this season. Gyokeres made a huge impact at Sporting since joining from Coventry City for €24m last summer, with 36 goals in 39 appearances. He has since attracted interest from several European clubs, including Arsenal and Chelsea, who are in the market for a new striker. Chelsea submitted a £73m bid to sign Gyokeres during the January transfer window, but Sporting refused to sell him and are holding out for a club to meet his £85m…