The attacking midfielder came on in the 68th minute of England's 1-0 win over Netherlands last week and was an unused substitute in the 1-1 draw with Italy at Wembley on Tuesday.
The 21-year-old had quickly established himself as a regular for the national team after bursting onto the scene with Spurs in the 2015-16 season.
"I have a problem because I have Marcus Rashford, Jesse Lingard, Raheem Sterling, Harry Kane, Alex Oxlade-Chamberlain and Jamie Vardy, so there is a huge choice," Southgate said.
"I don't foresee a scenario where any of those players don't come to Russia but I have to make those decisions.
"There is no reason he [Alli] will not come and he can make a huge impact there. He has the ability and I am looking forward to seeing him over the next few weeks."
With Kane injured, Sterling started both friendlies in a central attacking role, and Southgate was encouraged by the versatility of the Manchester City forward.
He said: "It was nice to see because if we play 3-4-3 we know Raheem can play the number 10 role, as he did against France last year, but could he play as one of two forwards when he usually plays on the wing for Manchester City?
"We know he likes playing through the middle and his performance reminded me of that little spell at Liverpool where he was behind Luis Suarez and Daniel Sturridge, running and dribbling through the middle of the park."
Despite the recent prominence afforded to Sterling, Southgate made it clear that Alli will not be easily discarded.
"We want competition for places but there was no ulterior motive to leaving him out," the manager said.
"We wanted to look at Raheem again but Dele is still an important player for us. He is in the mix."