By Simon Bird Eddie Howe has issued a firm warning to Arsenal and Euro rivals saying Newcastle will refuse to sell star striker Alexander Isak. The Newcastle boss could be in danger of losing one of his top two stars in Isak or Bruno Guimaraes this summer because the Toon will need funds to rebuild the squad. Sweden international Isak has netted 18 goals so far this season, the most in a top flight season since Alan Shearer in 2005, with Arsenal credited with an interest in a summer deal. But Howe today acknowledged the importance of Isak to Newcastle’s rebuilding mission and has warned off ri…