Jose Mourinho recently suggested the plan is for the 20-year-old to become the third-choice goalkeeper at Old Trafford for the rest of the season, while Sam Johnstone obtains some first-team football on loan with Aston Villa.
Pereira said on his deprtaure from Portugal: "I'd like to thank everyone at Belenenses who helped me to develop my game and become a better player.
"From the president to the coaches, all my teammates, I really appreciate the opportunity that was given to me. It was a great experience to be able to play for such a traditional club from the Portuguese league and be capable of helping the team to get good results.
"As I'm a Manchester United player, I need to do what my club asks and I'm anxious to return to Manchester and keep working hard with my team-mates."
Belenenses coach Quim Machado commented at a press conference: "It is not easy to lose the best players in a team like ours. We do not have the financial power of Benfica, Sporting and FC Porto. It's a shame for us. Joel took advantage of his opportunities. He has enormous potential."