Tottenham manager Mauricio Pochettino is now hopeful Alli could be involved in the Premier League this weekend, depending on how he comes through a final session on Friday.
"It's good news that he is going to be available," Pochettino told a pre-match news conference on Thursday.
"We need to wait until tomorrow and the last training session but he is doing so well and I hope he can play on Saturday."
Alli has scored seven goals in 25 appearances across all competitions this season, recording four assists.
Tottenham's injury problems are starting to ease up, with Harry Kane and Eric Dier both back in action in recent matches.
But Kieran Trippier missed the midweek win at Borussia Dortmund with a gluteal strain, while Harry Winks was replaced at Signal Iduna Park due to a groin injury.
Trippier will continue to undergo treatment, but Winks – who sat out last weekend's north London derby draw against Arsenal - could yet face the Saints.
"I think Trips, no [for Southampton]. He suffered a small problem and I think he will be out," Pochettino said. "Harry we will assess tomorrow."
Tottenham will be looking to recover from a three-game winless streak in the Premier League when they take on Southampton, following back-to-back defeats against Burnley and Chelsea, before a 1-1 draw at Wembley against Arsenal in the north London derby.
However, they did record a 4-0 aggregate victory over Bundesliga leaders Borussia Dortmund in the Champions League in that time, progressing to the quarter-final stage along with Manchester United, Ajax and Porto.