Mane agreed a four-year deal with Southampton before the transfer window closed earlier this month but has yet to arrive due to international commitments with Senegal.
Koeman's side return to Premier League action on Saturday against Newcastle United with no injury concerns following the international break, however the Dutchman remains uncertain whether his new signing will feature.
"I hope so," he told Thursday's media conference. "He had to travel to Ghana to get his stamp [visa] to get to England so it will be a late decision if he is available to play.
"He will travel [to England] tomorrow. If he is part of the team he will start on the bench.
"Sadio brings a different quality to the team, he is a player who is fast and has a good record for goals.
"We don’t have a lot of forward players so his qualities bring a little bit more, he is difficult for defenders to mark."
Toby Alderweireld also agreed a late switch to St Mary's Stadium before the window closed and Koeman expects the Belgium international to raise standards within his defence.
"We knew other teams were interested [in him] so I'm very happy he finally chose Southampton," he added. "He is a quality player.
"He has shown his qualities on the highest level possible. He will bring quality to the team and raise the level of the defenders."
