Barcelona boss Ernesto Valverde has lavished praise on Lionel Messi following the Argentine's brace against Manchester United in the Champions League on Tuesday night.

Messi scored twice in Barca's 3-0 win over United at Camp Nou, with the result seeing the Spanish champions advance to the semi-finals of the competition courtesy of a four-goal aggregate victory.

Valverde has claimed that the Argentina international is simply "unstoppable" in this type of form.

"Messi always steps up in these key games. When he is in this type of form, he is unstoppable," Valverde told reporters.

The 31-year-old has now scored 45 times in 42 appearances for Barca in all competitions this season, including 10 in eight Champions League matches.

Messi's best ever scoring season came in 2011-12, when he found the back of the net on 73 occasions in 60 appearances.