Peter Houston says possession is crucial if United are to cause upset DUNDEE United manager Peter Houston believes being clever in possession can be the key to his side upsetting Celtic in tonight's Clydesdale Bank Premier League match at Parkhead. United travel to face the champions on the back of a 3-2 win away to Kilmarnock at the weekend, but knowing they will have to up their standards even more if they are to get a win. Houston, who revealed last week he'll be leaving the Tannadice helm at the end of the season, insists they have got to keep the ball against Celtic. "It's another big test," said the manager. It will be a case of when we get the ball we have to keep it as much as possible. Ball retention is always important against a quality side like Celtic and it will be the same for this game. I'm sure that if we pass the ball like we can, we can do well. Kilmarnock get a lot of praise for being a passing team but I thought we did that very well against them at the weekend and I'll be looking for my players to continue that at Celtic. United came from behind to earn a last-gasp 2-2 draw with Celtic on Tayside back in November. However, Houston knows they are outsiders to pull off a shock in Glasgow, although he reckons that could work in their favour as they prepare to take on Neil Lennon's men. He said: "There will be a different level of expectancy against Celtic compared to what we had at Kilmarnock. We're probably not expected to get much because of the way Celtic are playing and they're at home. But sometimes these games don't always go the way they're expected to and this is a challenge for our guys. It's up to them to go out and show what they can do and give it their best shot on the night. I've got belief in my players and we know what we're capable of on our day. We want to go there and enjoy ourselves and hopefully have something to show for our efforts. United welcome back midfield pair Stuart Armstrong and Richie Ryan from injury, while defender Brian McLean is free from suspension. Houston said: "It will be good to have these guys back. We've had a pretty young bench for the past couple of games but this will hopefully help matters. We don't have the biggest of squads but we just have to work with what we've got.