The former Brighton and Hove Albion star had been tracked by the Black Cats, but Poyet decided against bidding for the target-man centre-forward because of the options that he already had at the club.
The Uruguayan boss said that he was happy going into the season with Connor Wickham, Steven Fletcher, Danny Graham and Jozy Altidore as his attacking options, but it looks a mistake to not have followed through with his initial interest in Ulloa now.
The Black Cats have struggled for goals this season and many of their striking department are already in danger of leaving the Stadium of Light.
Altidore has been linked with a January exit, Graham is out on loan at Wolverhampton Wanderers and Wickham's contract, which expires in the summer, has still not be sorted out.
It means that Poyet's options up front now look limited and next summer may see him having to focus solely on bringing in a striker with such a paucity of players in this department.
That could have been a problem which was solved by bringing in Ulloa and, even though the initial fee of £8 million that Leicester paid to buy him caught the attention, it seems that he has the ability to pay it back.
The 28-year-old already has five goals in his first 11 games for the Foxes and he looks a far better option that most in Sunderland's striking ranks.
Although Fletcher has done well since returning from injury, Wickham, Altidore and Graham have all fallen short of the standards required so far and their attacking threat would unquestionably have been increased with Ulloa in the team.
Poyet will have a chance to see exactly what he missed this summer at the weekend, when he takes Sunderland to face Leicester.
The match already looks like it could be a crucial one in both club's season and Ulloa will be hoping to make Poyet's error look even more costly by scoring for Nigel Pearson's side.