The 18-year-old Serbian Under-19 international had a spell on loan at Yeovil earlier this season and will bolster the Cobblers' faltering attack.
Manager Stuart Gray said: "In the last few games we have struggled for goals and by bringing Aleksandar in and having Adebayo Akinfenwa back from suspension we have some extra forward options for the forthcoming games.
"Aleksandar has scored goals at all levels and has played in League One this season. I saw him play during his time at Yeovil. He did well for them and played his part in a good run for the team, and hopefully he can do well for us.
"He has been training with us this week and I am sure he will fit in well with the squad. We are very grateful to Nigel Clough at Derby for allowing us to borrow Aleksandar. We saw Andy Todd benefit from a loan spell with us earlier in the season, and hopefully Aleksandar will be good for us and we will be good for him."