The Red Devils were comfortable 4-0 winners against League Two side Yeovil Town in the FA Cup fourth round on Friday night.
January signing Alexis Sanchez made his debut in the cup clash and the 29-year-old forward made two assists to make an immediate impression at Manchester United.
The Red Devils have won their last five games in all competitions to close the gap on runaway Premier League leaders Manchester City to 12 points.
Manchester United signing Sanchez could make his Premier League debut for his new club against Arsenal’s bitter rivals Tottenham Hotspur at Wembley on Wednesday night.
Spanish midfielder Herrera isn’t underestimating Spurs ahead of their trip to the English capital in search of a fourth successive Premier League win.
“I think we can be optimistic for the rest of the season. But we must know in the Premier League every team can beat you if you sleep for just one second so we have to be ready for the next game against Tottenham on Wednesday,” Herrera told Man United’s website.
Manchester United won the FA Cup in 2016 under Louis van Gaal following a 2-1 win over Crystal Palace in extra time at Wembley.
The Red Devils suffered a shock 2-1 loss to Championship side Bristol City in the League Cup quarter-final earlier this month.
The 20-time English champions haven’t won the Premier League title since 2013.
MORE: Man United latest news
MORE: Arsenal latest news
MORE: Chelsea latest news
MORE: Liverpool latest news