United face a short turnaround after Thursday's Europa League defeat at Feyenoord.
"In terms of recovery, it's not the best way, we all know that," he said of this week's schedule. "But, as I've told you before, we have a big squad. All the players in the squad want to show that we are here to help the team. It is not the best situation about the time between two games but this is football and we have to forget excuses, play and try to do our best.
"Watford, last season, were a very organised team, very tough to beat, and now they keep almost the same players. Maybe two or three changes but the rest are the same so it's going to be tough. Overall, if you are playing away in the Premier League you know there is no easy game. We won at Watford in the last minute last season. Hopefully, this Sunday, we can do it sooner. We want to win but it will be very, very hard.
"I don't know he [Quique Sanchez-Flores] lost his job," he added on MUTV. "Maybe he wanted to leave, but it's true the Premier League is getting tougher. Every season, every team wants to improve on last season. We want to do it as well so we have to fight for every game."