The veteran centre-back was signed on a free transfer by manager Nigel Pearson to add experience to Leicester's defence ahead of their return to the Premier League.
However, Upson is yet to make a competitive appearance for Leicester due to injury.
Pearson, though, confirmed on Friday that the 35-year-old is nearing a return to action.
"He's training with the first team now, which is good," he said ahead of Saturday's Premier League game at in-form Southampton.
"We have lots of options and an almost fully fit squad."
Leicester travel to second-placed Southampton on the back of a five-match winless league run, a return that leaves them in the relegation zone.
Pearson, who enjoyed a brief spell in charge on Southampton in 2008, has been impressed with Ronald Koeman's team and is fully aware of the difficulty Leicester face at St Mary's Stadium.
"I enjoyed my time at Southampton. It's a really great club in a great area," he added.
"They've done really well to be where they are. We know Southampton are a very good club. I have to say they've greatly impressed me with how they play and their quality."
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