The Scotland midfielder, 27, last featured for the Red Devils in the final game of the 2010-11 league campaign against Blackpool but had been sidelined for two months between early March and early May by the virus, which caused sudden weight loss.
He suffered a relapse over the summer but manager Sir Alex Ferguson confirmed this morning: "Fletcher is in the squad. He is back now."
He also revealed that defender Rio Ferdinand was back in training following a hamstring injury and that winger Antonio Valencia was training well following an ankle injury.
Ferguson expects captain Nemanja Vidic to be out for another month with a calf problem.
United face an Arsenal side which has taken just one point from their opening two league games and Gunners boss Arsene Wenger has found himself under unprecedented pressure in recent weeks, although their progress to the Champions League group stage in midweek has eased it.
Ferguson was astounded at the suggestion his great rival's position could be under threat, saying: "Who would replace Arsene Wenger? What he has done over the last 15 years... he is the best manager in Arsenal's history."
Ferguson was asked about the failure of Scotland's European representatives last night, with Rangers, Celtic and Hearts all eliminated from the Europa League to end the country's interest in continental club competition before the end of August.
"I bet you lot are loving it," Ferguson said.
"You had to ask, didn't you? It is disappointing."