You're basing your view on the current squad and not what the squad will be next season. If Messi arrives then Mahrez is redundant, old and on high wages.
it would be amazing if he got injured at all considering he's only played about 50% of our games since he arrived, he has an incredibly light work load. Does he start all big games? Not really, he didn't start Monchengladbach or Spurs or Chelsea, it's about 50/50.
In form he's excellent but if Messi arrives why would you use him? His work rate and defensive effort is really poor, so you certainly aren't going to play him with Messi unless you want to carry 2 players.
If the summer goes to plan, we'd have wing options of Messi, Sterling, Bernardo, Foden, Jesus, Torres and I don't see why you'd keep Mahrez with those options especially when there's probably a good market for him, he's 30 and eating up money.
He's not going to score goals like Sterling, he's not got the work rate that we might need to carry Messi like Bernardo, Foden or Jesus, he's not a long term young player like Jesus or Foden or Torres, he's not positionally versatile.
If we kept him and got Messi, we'd end up selling him in 12 months because he's unhappy at sitting on the bench for 80% of games and it'd be for 1/4 of the price we'll get this summer.