he Greek international striker Demis Nikolaidis, whose summer transfer to Spanish club Atletico Madrid from Athens-based AEK began humbly but has gained momentum in recent weeks, has been sidelined by a lower back injury that will need operating on. Nikolaidis, who has opted to undergo surgery in Athens either today or tomorrow, is expected to miss between four and six weeks of playing action. He had undergone the same operation several seasons ago. The Greek international was substituted an hour into his team’s clash against Real Madrid on Wednesday night after complaining about sharp lower-back pains. Nikolaidis’s team lost 2-0. The player felt hampered by the injury days ahead of the all-Madrid clash. Atletico Madrid began its quest in this season’s Spanish first division poorly, hovering about the league’s relegation zone, but the club has since rebounded to fifth place with Nikolaidis, a frequent scorer and leading performer for his club, widely acknowledged as a key component in his team’s impressive revival.