Manchester United medical staff remain unsure what caused Michael Carrick's nauseous spell on the sidelines but the midfielder will return to training this week.

Carrick has only played once this season - in September's 4-1 win over Burton in the Carabao Cup.

He complained that he felt sick and dizzy after the match and has experienced similar symptoms in training since, according to The Sun, who claim that United doctors are still baffled by the mystery illness.

Manchester United staff remain unsure what caused midfielder Michael Carrick's illness

Carrick, who watched United from the stands on Saturday, has been out since Septemeber

In addition to his one appearance, Carrick has been an unused sub five times this term, but has not even featured on the bench since United's 1-0 win at Southampton on September 23.

The 36-year-old former England star played in 38 games in all competitions last season, which saw him add the Europa League and a third League Cup to his list of major honours.

United boss Jose Mourinho would love to have a fit Carrick available right now as fellow midfielders Paul Pogba and Marouane Fellaini are currently unavailable.

Short on central midfield options, Mourinho switched to a 3-4-3 system for Saturday's 1-0 victory over Tottenham as Nemanja Matic and Ander Herrera were paired in the centre of the park.

United's lack of midfield options on the bench highlighted Mourinho's injury issues, with youngster Scott McTominay and the more attacking Juan Mata his only genuine back-ups for Matic and Herrera.

Mourinho's Red Devils can secure their place in the knockout phase of the Champions League by beating Benfica at home on Tuesday night,

United won the reverse fixture 1-0 earlier this month when Marcus Rashford profited from a goalkeeping howler to score the only goal.

The 36-year-old had felt sick and dizzy after playing in a Carabao Cup win over Burton Albion