Assistant referee Constantine Hatzidakis ruled Manchester United midfielder Juan Mata to be offside in the 18th minute.
Striker Romelu Lukaku saw his shot parried by Leicester goalkeeper Kasper Schmeichel into the path of Mata, with the Spaniard smashing home the follow up.
But Hatzidakis claimed he was beyond the last defender, Foxes captain Wes Morgan.
Initial replays suggested Mata was onside, but after a line was drawn on the screen, it was clear Mata’s right foot was offside.
“Yeah they did get it right,” Foy said on BT Sport.
“It appeared on first viewing that Juan Mata was onside and it was a really tough call.
“You can see the lines across the pitch now he was fractionally off.
“A really tight decision but an accurate one and congratulations to the assistant for making it.”
Commentator Owen Hargreaves initially thought it was offside until he saw the second replay.
“It’s so difficult for these linesman to see that in real time,” the former Manchester United midfielder said.
“It’s difficult to see on the replay when you stop it.”
The score at Old Trafford was 0-0 at the time of the incident and remained like that until half time.
Substitutes Marcus Rashford and Marouane Fellaini put the hosts two up in the second half after Romelu Lukaku missed a penalty.