Indeed, the likes of Andreas Pereira, Jesse Lingard and Juan Mata have struggled to contribute this season, and it would be a surprise if the Red Devils were not fairly active ahead of the 2020-21 campaign.
Villa are facing the possibility of being relegated from the Premier League this season, which could, in theory, make it easier for United to sign Grealish during this summer's transfer window.
The situation with Maddison is slightly more complicated, though, as Leicester are in a brilliant position to secure Champions League football for next season, providing that the current campaign is completed.
Here, Sports Mole looks at how the duo stack up against each other during the 2019-20 season.
There is nothing to separate the two players in terms of goals in all competitions this season. Indeed, both have managed nine, although Grealish has only played 31 times this season to Maddison's 35.
Grealish has outscored Maddison in the Premier League this season seven to six, meanwhile, and the Villa captain boasts a record of 0.27 goals per match to Maddison's 0.21 per match in England's top flight this term.
Interestingly, neither has scored in the Premier League with their head this season. Six of Grealish's seven goals in the league have come from inside the box, while Maddison has managed four from outside the box.
What's more, six of Grealish's Premier League goals have come while operating or at least starting as a left-sided forward, while all six of Maddison's have come from a central position.
Neither player has scored more than once in a league match this term, while they have netted from just one free kick between them this season - Maddison's set piece against Southampton back in October.
Grealish has a superior assist record to Maddison this season, managing eight in all competitions to his fellow Englishman's three. Six of Grealish's assists have actually come in the Premier League, while Maddison has only managed three assists in 28 Premier League appearances this term.
Maddison has created more big chances than Grealish, though, seven to six, while the Leicester attacker has provided 19 though balls to Grealish's two; that statistic, however, is not surprising considering that Maddison operates behind Jamie Vardy while Grealish does not have a clear runner to hit at Villa.
Grealish has the better pass completion - 84.4% to Maddison's 83%. The latter boasts 390 passes forward to Grealish's 300, though, suggesting that the Foxes attacker is the more attack-minded in general play.
In the Premier League, Maddison has had the most shots - 62 to Grealish's 55, although both players have identical records for shots on target this term (17). Grealish has posted 27 shots off target to Maddison's 21, though, meaning that the latter has a better shooting accuracy of 44.7%.
Maddison's touches in the Premier League are also a lot higher having managed 1,972 to Grealish's 1,691. Meanwhile, Grealish has been dispossessed on 46 occasions to Maddison's 28. The crossing statistics are also in Maddison's favour 202 to 99, suggesting that the 23-year-old offers more in general play, particularly in a wide position which is a surprise considering the positions of the two players this term.
Neither player has picked up a red card in the Premier League this season; Grealish has more yellow cards (7) than Maddison (4), but the Leicester attacker has committed more fouls - 33 to Grealish's 22.
It is also certainly worth noting that Grealish wears the captain's armband for Villa, naturally placing more responsibility on his shoulders. Maddison is certainly a key player for Leicester, although the attacker is helping his team chase different goals at the other end of the table.
For all the criticism that has been aimed at United this season, Ole Gunnar Solskjaer's side have kept nine clean sheets in their last 11 matches in all competitions.
There is no question that the Red Devils have become tougher to beat in recent months, while their performances against the so-called bigger clubs this season have been impressive.
Grealish has been involved in four clean sheets for Villa this season to Maddison's seven for Leicester, while the latter has been part of the side that has conceded 25 times in the Premier League - during his time on the field - in contrast to Grealish's 52, not counting matches where he has not played.
Grealish has made more blocks (11) and interceptions (14) than Maddison this term, although the Foxes attacker has both made (53) and won more tackles (25). Grealish has also won 16 aerial battles to Maddison's seven, but the Villa captain has one error leading to a goal while Maddison has zero.