Sam Allardyce is unsure if Manchester City loanee Mangala (knee) will play again this season having picked up a problem against Crystal Palace, while Coleman, also injured in that game, is struggling with a thigh issue.
"Its a very difficult thing to assess just how long its going to take, whether its going to be longer than first predicted or not," he said of Mangala.
"At this early stage, while people put a lot of times on how quick youre going to recover, it can never actually really be the case. It cannot be a certainty that it can be eight, 10, 12 weeks at this early stage. You have to judge it week by week."
Watford have won just one of the 10 Premier League meetings between the two sides, although this did come at Vicarage Road in December 2016 when the Hornets beat Everton 3-2.
Manager Javi Gracia has urged the clubs fans to get behind the team and create a similar atmosphere to their 4-1 win against Chelsea, saying: "We try to go on the same way [as we did against Chelsea].
"We got the result with the support of our fans. The next two games we need them as after we have some difficult matches away. It is important the supporters help the team in the next game. I think with the backing of our support we will be able to achieve our objectives."
Although Mangala and Coleman will be unavailable, Leighton Baines is likely to return to the squad for the first time since November after a calf injury.
However, Ramiro Funes Mori (knee) will play for the U23s again before being considered for Allardyces senior team.
Watford expect to have goalkeeper Heurelho Gomes available again following a back problem, and Miguel Britos (ankle) and Kiko Femenia (hamstring) could also return, along with Will Hughes (hamstring).
Stefano Okaka is also expected to be drafted back into the squad, but Nathaniel Chalobah (knee), Younes Kaboul (foot), Isaac Success and Craig Cathcart (both knee) remain unavailable.
