Wait, wait. It's possible that Ash faces Virgil after Kotetsu: READ THIS BEFORE JUMPING INTO CONCLUSION.:
Unova League Conclusion!!! Pikachu vs Lucario
"The Isshu League fiercely continues into the fourth round. Ash and Kotetsu's battle continues to become even more extreme. Kotetsu's partner Riolu then evolves into the Wave Pokemon Lucario. With all of its moves powered up plus learning a new move, Satoshi and Tsutarja are both cornered..."
Unova League Conclusion means that the league is ending.
Pikachu vs Lucario means that one of the match-ups is Ash vs Lucario.
And about the summary herself:
We know that Lucario evolves during last episode. Lucario appears in both the December and January calendar and the summary of BW S2-23 implies that Riolu will evolve in the last episode.
Lucario vs Snivy could be the cccllliiifff-hhhaaannngggeeerrr (triplicated because we wait 3 weeks) of the episode.
We know that Lucario AND Pikachu battled before in this battle, meaning that the battle will be relatively quick.
What really happens is that Ash vs Kotetsu ends at minute 4-5 of this battle and immediatly afterwards Ash faces Virgil. The Virgil battles ends in the minute 18 of the episode, the champion wins in 30' and the rivals leaves in 30'. End of the league.
Remember: The summary of each episodes is evading every possible spoiler (not between episodes, fortunately for us). The Newtype summary is possibly only representing the cliff-hanger between the two episodes.
EDIT: The Hoenn battles with Morrison and Tyson has 15 minutes of overall battle (the rest are cliff-hanger, TR flashbacks, etc) Ash vs Virgil could be made in 13-14 minutes of screentime.
And remember that Virgil is hyped as a rival of the league. Makes more sense that Virgil faces Ash than Virgil faces Kotetsu. All the media before hints (but not confirms) that Ash is facing Virgil last in the league.
Remember: This is Newtype summary. This isn't the manga scans (they DON'T GIVE SPOILERS directly). We know that Kotetsu is going to win or lose if these summaries that comes from:
-Pokemon Fan, Dengeki, TV Tokyo summary, Next Episode preview and Famitsu only features Ash vs Kotetsu in the summaries. Furthermore, we have the end of the 20th December that we can see if Lucario vs Snivy is the cliffhanger or not.