The young Hornets striker agreed a season-long loan move back to the League One club. He spent the second-half of last season with Wimbledon.
Downes told the club website: “I am just really pleased to get Michael back and I'm looking forward to him joining up with the other strikers. He was a big part of what we did last season.
"Michael is a forward with a lot of pace who gets in behind defences. After having him here previously, we know that he has a great attitude and he puts pressure on defenders."