Pards is desperate for the centre-back to prove his fitness as his performances have been crucial as Reading embarked on a winning run.
The 23-year-old suffered a serious gash to the head during last weeks win at Gillingham, prompting Pardew to suggest at the time that he wouldn't be fit for the Sheffield United match.
With Upson touch and go Pards will wait until the last minute before naming his team.
The Royals boss told readingfc.co.uk: "I won't be able to name my squad until near kick off. This season I've got a lot of time to prepare my chosen team for a game, but this time it's different.
"Matthew Upson will be one of those who is right up to the deadline, but in John Mackie we have the perfect replacement.
"It's going to be a tough game for us but hopefully we'll be able to keep our run intact. The key is not to defend too deeply, and we must control the game.
"Against Gillingham, Walsall and Grimsby we controlled the game throughout and that is what I want from this weekend's game."
Tomorrow's game with the Baldes is crucial as the Yorkshire side are hot on the heels of the sixth placed Royals.
United this week added to their squad by capturing Dean Windass on loan from Middlesbrough. The Boro star could make his debut tomorrow.
Pardew commented: "They have invested heavily to get into the top six, which they haven't achieved yet, while we have got ourselves into a great position. Windass is a fantastic player but it's his first game which is going to be difficult for him."
Meanwhile, a move to bring Upson to the club on a permanet deal appears unlikely as thge Gunner's centre-back is wanted by Permiership giants Manchester City.