In a game played in soaking conditions the opening goal arrived after the only quality piece of play in the first half, as Nacho Cases curled in a wonderful cross from the left that Carmelo Gonzalez just had to redirect past Andres Fernandez.
Gregory Arnolin should have sealed all three points but he somehow headed wide from inside the six-yard box and that miss to prove crucial as Osasuna equalised against the run of play when Marc Bertran's cross from the right neatly headed home by Lekic.
Inaki Tejada took charge of Sporting for the first time after Manuel Preciado's sacking in midweek and his influence appeared to have galvanised the players as they pressured Osasuna intensely, but at times they lacked the quality to make their dominance count.
Cases wasted their best opportunity of the opening quarter as he fired straight at Andres, but the opener did arrive just after the half hour as Cases' ball into the heart of the area found Carmelo arriving between two Osasuna defenders to prod home.
Osasuna boss Jose Luis Mendilibar introduced Nino after the break and the veteran striker gave the visitors much more of a threat in the final third as his first effort was well-stopped by Juan Pablo.
Andres then did well to turn Miguel de Las Cuevas' long-range shot behind, and from the resulting corner Gregory was left all alone inside the area but the defender headed just wide.
A minute later Nino nearly made him pay as he got in behind the Sporting defence but his deflected effort was again turned away by Pablo.
And just as Mendilibar was ready to introduce Ibrahima Balde for the ineffective Lekic, the Serbian scored his first league goal of the season after poor marking in the Sporting defence allowed him space to head in Bertran's cross.