The 30-year-old Argentinian forward has not featured for the Hornets first team since suffering a serious cruciate knee ligament injury against West Ham back in February.
However, following a recent 45-minute run-out in a behind-closed-doors game against Millwall, Zarate managed to get 80 minutes under his belt in the victory over the Clarets.
Performance director Gavin Benjafield said on the club's website: "Mauro's rehabilitation is going really well. As well as full team training, he's been exposed to two game situations and he is looking very confident.
"He's still continuing to work individually on his fitness and preparation work for the remainder of the season, but he has made really good progress."
Boss Marco Silva could also have Roberto Pereyra back in his plans soon as he is recovering well from the muscle injury sustained in the opening game against Liverpool a month ago.