Chelsea midfielder Deco is likely to be out for the rest of the season after suffering a recurrence of the hamstring injury that has plagued him for the last few months.
Deco was substituted in the first half of their 1-0 win over Manchester City at Stamford Bridge and Hiddink admitted he does not expect him to return before the summer.
“He was fit and worked hard, but after 20 or 25 minutes he had this problem again, so he will be out again for a while,” said Hiddink.
“I think Deco will not be recovered very shortly. He’ll have to be very fit to come back this season. I have my doubts to be honest, but let’s see what happens.”
Hiddink, though, was looking forward to an exciting title run-in after Chelsea recorded their fourth straight league win since he replaced Luiz Felipe Scolari in January.
He has yet to taste defeat since agreeing to become the club’s interim coach until the end of the season.
The dutchman is still wary of United’s game in hand but was boosted by Liverpool’s 4-1 demolition job on Sir Alex Ferguson’s side at Old Trafford.
“If it was just four points it would be very close, but they have that game in hand,” he said. “But it’s clear that after Saturday’s unexpected result that the tension has come back in the league.
“It’s a boost. But if we want to track them, we have to keep on winning. That’ll give tension to the end of the season, which is good for everyone.
“I don’t know what Manchester United’s reaction will be. When you are in the driver’s seat and someone else is coming, you can get a little bit nervous. It is a little bit open now.”