Alexis Sanchez's frustration at Arsenal's failure to qualify for the Champions League last season was central to his decision to join Manchester United, says Jose Mourinho.

Chile forward Sanchez completed his move to Old Trafford last month in a swap deal that saw Henrikh Mkhitaryan join the Gunners.

Sanchez is set to make his European bow for the Red Devils when United tackle Sevilla in the first leg of their last-16 Champions League tie on Wednesday.

Mourinho described the former Barcelona star as a player United "needed" and believes the 29-year-old's desire to play in Europe's premier competition was the motivating fact for his move.

"It is true that we have a new player, Alexis Sanchez, and we could have waited for the transfer market in the summer but we decided to go now in the transfer market," Mourinho said.

"We felt we needed him. Manchester United in the Champions League is quite a big thing, Alexis came with all his efforts to play on the Champions League stage.

"We know it has been hard for me as a coach to motivate my players for a competition we didn't want to play [the Europa League]. But because we won it we got to qualify for the Champions League so motivation is always there - we always want to play against the best teams.

"So Alexis wanted to come to a team to play in the Champions League."

Mourinho has won the Champions League as a coach with Porto and Inter, and enjoyed European success with United in the Europa League last year.

And the Red Devils boss feels his current squad is somewhere in between the sides he coached to success on Europe's biggest stage in terms of experience, but suggested his side are not among the favourites for the tournament.

"I won with Porto, very young, inexperienced team at this level, so many playing the Champions League for the first time," he added.

"It was the opposite at Inter, lots of over 30s with lots of players with great experience at high level but not successful. 

"This squad is more in the middle a bit. We have some players with experience and others without experience but I think in the end I normally say Champions League dreams they start around the quarter-finals and not in last 16, it still looks a long way to go. 

"When a team reaches the quarter-final, this is the moment. Even when they are not favourites, like us, they start realising anything is possible."