Long went off during the second half of Saturday's 2-1 defeat at St Mary's, having opened the scoring against the Blues.
The forward has netted ten goals for Saints this season, including a strike in each of the last two games, and Koeman said he will be assessed this afternoon.
"It was a little bit cramp, it was not really a hamstring injury. We have the session later on and then we will make the decision if he is ready for tomorrow, but he will be doubtful."
Koeman is pleased, however, with the options he has at his disposal if Long can't make the game, with his bench on Saturday having featured a significant attacking threat, including Sadio Mané and Graziano Pellè.
"Shane is doing very well this season," said Koeman. He's very dangerous, he's scoring goals, he's creating a lot of problems to defenders. You like to have everybody available, but we have real competition in all parts of the team and that makes the team stronger.
"If you can't use one player then the other player needs to be sharp to play and to show his own qualities."