However, Craig Brown wants to create an upset at Hampden Neil Lennon has told his players to expect a much-improved Aberdeen side from their previous Hampden encounter as they meet in their second semi-final of the season. Celtic were 4-0 up and cruising against the Dons at half-time in their Co-operative Insurance Cup meeting in January before a solitary Aberdeen second-half goal provided them with a small portion of respectability. Rangers needed a spectacular goal from Nikica Jelavic in midweek to see off the Dons which gives Celtic further evidence of the threat awaiting them in the semi-final today. Lennon said: "We know we have a hard game in front of us. By all accounts Aberdeen played very well on Wednesday night. There is something to play for there, a piece of silverware that means a lot to the club and myself. Semi-finals are notoriously difficult anyway, quite tight and tense. We played very well in the League Cup semi-final and to be 4-0 up at half-time was a huge bonus. We are probably not expecting that to happen on Sunday, we will have to be patient and mentally strong.'' Celtic are 17-1 up on aggregate in the four meetings with Aberdeen this season but Lennon knows opposing manager Craig Brown and assistant Archie Knox will have their players determined to change that dramatically. Lennon said: "Every game is different. It's a big game for Aberdeen in the context of their future and how they go about next season. It will mean a lot for Craig and Archie for their team to put on a performance after the disappointment of the League Cup game. So we will have to prepare as best we can for that." Another warning for Celtic comes from last season's semi-final when they lost 2-0 to Ross County in one of the greatest shocks of the tournament's history. Lennon went on: "You can use it as motivation to make sure it doesn't happen again. The guys who played last year will want to eradicate that memory and put it to bed.'' Aberdeen manager Brown believes his side can play free of pressure because all the expectation is on Celtic. Brown knows Celtic will be expected to pick themselves up from disappointment on their last trip to Hampden Park - the Co-operative Insurance Cup final loss to Rangers last month - with a fifth win of the season over the Dons. He said: "We've nothing to lose. This is a particularly big game for us - but it's an even bigger game for Celtic. If they slipped up in the league and if they don't beat us and get to the final in the cup it could be a major disappointment for Celtic. They're under a little bit of pressure because they're expected to beat us, they're expected to get to the cup final. They're also top of the league and there will be great expectation for them to win that. If they faltered in both competitions it would be a major disaster. It's not a disaster if we don't get to the cup final, a disappointment only.'' Brown believes Celtic have the best squad in the country. But the former Scotland boss is excited by the challenge that poses and believes his side can overcome Lennon's men. He added: "They've got terrific options, Celtic. More than any other team in the SPL, they're harder to predict. But I'm an optimist and we've got a mentality in the players that we're positive. We've got a determination and a belief that we're capable of going and causing an upset.'' Key to the Dons' hopes is the team Brown can put out on the Hampden pitch. Injuries have afflicted Aberdeen this season, with Rory McArdle (shoulder) and David McNamee (hamstring) recently joining long-term victims Yoann Folly, Darren Mackie and Fraser Fyvie in the treatment room. Captain Paul Hartley, though, could return after missing the Dons' last five games with a knee injury, while Steven Smith (calf) is a doubt. Brown is set to make a late decision on whether to include former Celtic and Hearts midfielder Hartley and while he would prefer his most experienced player to play, he will not be taking a chance. The Dons manager said: "He's desperate to play, but he's got enough experience to know when he's taking an unnecessary risk. The final decision will be Paul's in terms of fitness, it will be mine in terms of selection. You don't want to take a risk with him and put him out. We've got another minimum five matches after this one, hopefully six."