Arsenal manager Arsene Wenger has explained the reason behind why he substitutes certain players.
The Frenchman has become known for not making any personnel changes until the final 20 minutes of a match.
Alexandre Lacazette rarely finishes a game; the forward has only completed a full 90 minutes on seven occasions this season.
"We know the players' physical qualities and who we expect to drop in the second half or not. Nobody's tired enough at half time to be substituted for that, but in the second half you'll see that sometimes this player will drop in the last 20 minutes," Wenger told Arsenal Magazine.
"We have a fatigue factor coming in at different levels for everybody and we know the players who drop more in the second half than others. Some remain completely consistent and some even push upwards in the second half.
"There are players who drop up to 15 per cent in the second half, so you have to consider that in your substitutions. Even if you do make substitutions, you always try to keep one free because you know this guy could need to be substituted as well so you need to keep a place for him."
The Gunners are currently preparing for this Sunday's Premier League match against Stoke City at the Emirates.
