Carlo Ancelotti is reportedly considering starting David Alaba at left-back against Arsenal for the first time this season, with Ferland Mendy out.

COPE journalist Miguel Angel Diaz reports that Carlo Ancelotti is weighing up the possibility of playing David Alaba at left-back against Arsenal on Tuesday, with Ferland Mendy set to miss the match.
Mendy has been Ancelotti’s favoured option at left-back in the Champions League this season, with the 29-year-old starting 11 of Real Madrid’s 12 games in the competition so far. The only previous match he didn’t start was a League Phase game against Atalanta, which he missed due to injury.
The obvious alternative to Mendy is Fran Garcia, who plays at left-back for Real Madrid on a regular basis in La Liga. Though he’s only started one Champions League game, he’s started 20 games in domestic competition this season.

By contrast, Alaba hasn’t started a single game at left-back this season. He’s missed large parts of the season with an ACL injury, but even when available he’s played at centre-back.
It would be a surprising move from Ancelotti to put Alaba on the left for this match, but the Austrian international has played the position for many years throughout his career so far. He certainly has the experience to handle the occasion.

As for why Ancelotti might be considering the switch, it’s likely he’s aware that Arsenal’s biggest creative threat comes down the right-hand side, with Bukayo Saka, Martin Odegaard, and either Jurrien Timber or Ben White.
With Garcia lacking experience in these big games and Mendy absent, Alaba is probably the safest option to try and shut down Arsenal’s right flank.