The Czech winger finished his second Hearts spell at the end of last season but had been training with the club with a view to signing a new deal.
However, the Scottish Premier League have imposed a signing ban on the Edinburgh outfit due to their late payment of players.
Instead, he has signed on at United until January and told the club's official website: "For both the club and myself it is a good deal. In January, we can then look at things and see if it suits us both to extend the contract.
"Within 10 minutes of meeting the manager, I had decided I wanted to come here. It was an easy decision.
"I have kept fit training with a few of my previous clubs over the last few months so it is just match fitness I have to get up to speed with."
The 33-year-old has also represented Southampton, Hertha Berlin and Marseille during his career.