"Andrey has been here since minute one of pre-season, and he's been amazing. You can really feel the progress he's making already," Maguire said of Santos, who joined from Chelsea for a fee reported between £48m and £50m on a deal running to 2031 with the option of a further year.
Santos has featured heavily in United's pre-season programme under Michael Carrick, including starts against Wrexham and Paris Saint-Germain, having already impressed the manager with his passing range from a double-pivot role.
Tielemans has since joined him in Carrick's midfield, making his own United debut off the bench in last weekend's 1-1 draw against PSG after signing from Aston Villa for £35m once the club triggered his release clause. His involvement had been delayed by Belgium's run to the World Cup quarter-finals, where he captained the side and scored twice before a hamstring injury forced him off against Spain, plus the standard rest period afforded to players returning from the tournament.
Maguire, who played alongside Tielemans during a short spell together at Leicester City, backed the 29-year-old to make an instant impact once fully up to speed.
"Youri, I played with him at Leicester for a few months, so I know the ability and talent that he has," Maguire said.
"He's at an age now where he's going to come in, I'm sure he's not going to need settling in, he's going to come in and be a big player for this club. He's a top player, I know that."
Manchester United have committed close to £85m combined to the pair as they rebuild a midfield that lost Casemiro, part of a rebuild that also saw an agreed £35m move for Atalanta's Ederson collapse over medical concerns.
Maguire, who turns 34 in March, remains part of Carrick's leadership group as United - back in the Champions League after finishing in the top four last season - continue their preparations for the visit to newly promoted Hull City on the opening weekend of the Premier League season.