The Dutch international missed the Hornets' trip to the Hawthorns last week after clashing heads with Adrian Mariappa in training and suffering a deep cut to the face.
The medical team ruled him out of the trip to the trip and he is still wearing protective face equipment. He will be required to wear protective head gear in the short-term, but has been cleared for action and joined training again this week.
Janmaat, who signed from Newcastle in the summer, has been back in training and is looking more likely to be involved against Everton. Juan Camillo Zuniga filled in last weekend and put in a decent performance, however Mazzari is almost certain to recall Janmaat if fit.
Dutchman expected to return to Hornets' starting eleven
The reliable full-back has been a stalwart for boss Mazzari since he joined the club for £7 million in the summer and Zuniga’s decent showing is unlikely to change that.
Watford will also be able to call on centre-half Miguel Britos and midfielder Valon Behrami after the pair served their respective suspensions.
However, Janmaat’s fellow defender Younes Kaboul is unlikely to feature against Everton as he has not recovered from the muscle injury which saw him miss Saturday's 3-1 defeat at West Brom.
The other notable absentee for Mazzari is midfielder Roberto Pereyra, who will be a big miss for the Hornets as they look to recover some form.
He was dismissed controversially against West Brom after a scrap with James McClean and would have been beginning a three-match stint on the sidelines, had his ban not been reduced to one game on appeal.
Watford will be looking to get back to winning ways on Saturday after two disappointing defeats, whilst their visitors are enduring a torrid run themselves, with just one win in ten for Ronald Koeman and Everton.