United are expected to target at least one new midfielder, with Casemiro set to depart at the end of his contract and uncertainty continuing around Manuel Ugarte’s long-term role.
Stiller, who has impressed in Germany this season, is among several names being monitored, alongside Carlos Baleba, Elliot Anderson, Bruno Guimaraes and Adam Wharton.
The 24-year-old was already linked with a move to Old Trafford last summer, but United prioritised attacking reinforcements instead, bringing in players such as Bryan Mbeumo, Matheus Cunha and Benjamin Sesko.
However, any renewed move for Stiller now looks more complicated due to a unique clause in his Stuttgart contract, reports Sky Germany.
The Bundesliga side can activate a €40 million (£34m) release clause themselves for just €2 million (£1.7m), effectively removing the fixed fee and placing full control of any future transfer in their hands.
Crucially, the clause can be triggered at any time and cannot be rejected by the player, meaning Stuttgart can ensure Stiller remains tied to the club without a defined exit price.
If activated, his current deal — which runs until 2028 — would remain unchanged, but any interested club would then need to negotiate directly with Stuttgart, likely driving up his value significantly.
This scenario presents a major obstacle for Manchester United, who would no longer be able to rely on a release clause to secure a straightforward deal.
Stiller’s performances have not gone unnoticed, with Stuttgart sporting director Fabian Wohlgemuth previously acknowledging growing interest while downplaying immediate speculation.
“He has a contract with us. It's far too early to speculate about that,” he said.
“Of course, one or two clubs will have had their eye on him. He's having an outstanding season.”
Wohlgemuth also hinted at the midfielder’s potential international future, underlining his rising profile.
“Maybe he'll even play in a World Cup, we’ll see. We're happy he's with us.”
For United, the message is clear — while Stiller remains a target, Stuttgart currently control the situation, and any deal will require significant negotiation rather than a simple release clause trigger.