The Blues will start the Premier League weekend in fourth place in the table and two points ahead of Arsenal in fifth spot.
Chelsea FC played out a 1-1 draw with Bundesliga side Eintracht Frankfurt in the first leg of their Europa League semi-final on Thursday night.
The west London outfit will have two days to regroup before their London rivals Watford make the short trip to Stamford Bridge.
Maurizio Sarri’s side were 2-1 winners against the Hornets in their meeting at Vicarage Road during the festive schedule.
Former Arsenal midfielder Merson is confident that Chelsea FC will beat Watford in their London derby on Sunday afternoon.
“I think Chelsea will win,” Merson told Sky Sports.
“They have to win, if they do United are out of it and that’s it. If someone says you’ve got to beat Watford at home to get in the Champions League, you’d take that.
“There’s not been a lot in Watford’s games recently, they’re a hard team to play against but you’re sitting there thinking have they got one eye on the FA Cup final?”
Chelsea FC are looking to finish in the Premier League’s top four for this term after Antonio Conte could only lead the Blues to a fifth-placed finish in his second season at Stamford Bridge.
Sarri’s men can still reach the Europa League final if Chelsea FC beat Eintracht Frankfurt or play out a goalless at Stamford Bridge before emerging victorious against Arsenal or Valencia in the showpiece.
Chelsea FC lost to Manchester City on penalties in the League Cup final earlier this season.
MORE: Man United latest news
MORE: Arsenal latest news
MORE: Chelsea latest news
MORE: Liverpool latest news