Jose Mourinho downplayed Manchester United's chances of winning the Champions League ahead of their long-awaited return to the competition.Having last season missed out for just the second time in 22 years, a return to European football's top table was secured by winning May's Europa League final against Ajax.Mourinho believes the Champions League is the "natural habitat" of the three-time European champions, who kick-off Group A against Basle at Old Trafford on Tuesday evening.It will be United's first match in the competition for 644 days and appears a great opportunity to hit the ground running, although the Portuguese knows quite a challenge lies in wait in this competition."I think motivation is very important and of course motivation is very high," United boss Mourinho said."I think in the Champions League there are four or five teams with an incredible level of quality, of experience, of know-how and that's what makes the difference."Apart from that, there are many teams in the Champions League that are very similar to teams in the Europa League."In fact, Feyenoord and Anderlecht, who we played them last year in the Europa League, were champions in their countries and now they are playing in the Champions League, so there are many teams with similar qualities."We have to try to qualify, we have to try to go to the last-16 and enjoy that special knockout phase with the best teams in Europe."While the likes of Real Madrid, Barcelona, Bayern Munich and Juventus are sure to make life hard, few expect United to slip up in a group containing Basle, CSKA Moscow and Benfica.Their impressive start to the Premier League has only furthered that belief, with Mourinho speaking of the confidence coursing through his squad ahead of Tuesday's Group A opener.The Portuguese is also boosted by the likely return of Marouane Fellaini - a divisive figure whose midfield presence was missed in Saturday's 2-2 draw at Stoke.The re-occurrence of a calf complaint sustained on international duty was behind his absence and appeared to grate on Mourinho as Fellaini played Belgium's match against Greece despite the issue rearing its head beforehand.The 29-year-old missed training on Sunday but returned to the group on Monday, when Marcos Rojo, Luke Shaw and Ashley Young were among others in action on their return from long-term injuries."He's a very important player for me - much more important than what you can imagine," Mourinho said. "I feel weaker without Fellaini in my squad."It doesn't matter if it is on the pitch or if it is on the bench, so if his conditions improved he will be selected because I need him. But in this moment I cannot say yet."David De Gea will start ahead of Sergio Romero in goal, while Chris Smalling and Victor Lindelof will come in for suspended Phil Jones and Eric Bailly."It is not an opportunity, it is just normal," Mourinho said of his centre-back changes. "I don't think Bailly and Jones are better than them. I think they are the same level, they are just different players."Even if Jones and Bailly were not suspended, probably I would still play Lindelof and Smalling tomorrow because for me they are the same level. It is just a question of option."