Long boosted Saints’ strike options this week, when he arrived in a £12.5m deal from Hull, and he is available to feature in the Premier League opener at Liverpool.
Romanian international centre-half Gardos, who also arrived on Thursday, in a £6m transfer from Steaua Bucharest, will not be involved, though.
“He is not in the team for this Sunday, because he had an injury four or five weeks ago, a hamstring one, and he’s still not on a 100 per cent physical level to play,” said Saints manager Ronald Koeman.
“He needs maybe two or three weeks more for training and to be ready for playing.”
Koeman was also delighted with the capture of Republic of Ireland international Shane Long.
“First of all, we are very happy that we signed Shane Long, because from the beginning of pre-season it was a target on our list of players to bring in,” he said.
“Shane Long is an important player of the team and we need that type of striker because we didn’t have that type of striker in our squad and we are very happy to sign him.”
Long has not had much time with his new teammates yet, but Koeman said he will make a final decision on what role he will play at Anfield once the squad have conducted their final training session today.
If he does feature, he will be one of a number of players making their competitive debut for Saints.
Fellow summer signings Dusan Tadic, Graziano Pelle, Ryan Bertrand, Saphir Taider and Fraser Forster are all expected to be part of the matchday squad.