Chalobah has made 10 appearances in the league for Chelsea.
Chalobah, 22, spent the 2012/13 season on loan at Vicarage Road.
Raft of incomings and departures always expected at Vicarage Road.
24-year-old has never made a senior appearance for United after nine loan spells in six years.
England international is out of contract in 2018 and is eager to leave to boost World Cup hopes.
Sought-after Portuguese boss joins the Hornets just days after leaving relegated Hull City.
Hull were one of the three clubs to be relegated from the Premier League this season.
Red Devils out to win a trophy they have never won in their entire history.
City thump depleted Hornets to seal third place, while Liverpool cement fourth spot ahead of Arsenal.
Manchester City need a win to secure third place.
Chairman Scott Duxbury says decision was reached after talks over the club's future goals and aspirations.
Terry has already announced his decision to leave Chelsea after his contract expires in summer.
Premier League champions out to celebrate success.
Klopp's men go some way to cementing top four spot.
Third place Liverpool will look to consolidate their place in the top three of the Premier League.
England internationals could both feature against Watford on Monday [1 May].
Lallana picked up an injury in England's World Cup qualifier against Lithuania in March.
Spurs looking to cut Chelsea's lead at the top to four points.
Tottenham have won their last five games in the league.
Tottenham and Leicester also in action during the 3pm kick-offs.
Cleverley was quickly frozen out at Goodison Park under Ronald Koeman
Cleverley joined Watford on loan from Everton in January.