Januzaj glad to get game time against Foxes
The Manchester United midfielder was handed just a fifth Premier League start of the campaign and will hope to retain his place for Tuesday's FA Cup fourth round replay against Cambridge United.
Robin van Persie and Radamel Falcao were on target, with Wes Morgan's own goal completing a dominant first half for the hosts, while Leicester responded through Marcin Wasilewski after the break.
"I haven't played too much, so it was good to get 90 minutes," he told MUTV after the victory at Old Trafford, which exacted revenge for September's 5-3 loss at Leicester.
"I was tired and had cramp in the last 15 minutes, but I knew I had a job to do.
"We played well, especially in the first half. We fought for each other. The second half wasn't as good, but we’re happy with the result.
"We were good going forward and could have scored more than three goals, but hopefully we can take this into Tuesday night [against Cambridge] and win the game.
"We played at a higher tempo, especially in the first half. We moved the ball quickly and our finishing was also good. We went into the break 3-0 up, which was great. Although we should have scored more goals, at least we have the three points and we’re all happy with that."