The 26-year-old was United's leading marksmen last year with 35 goals in all competitions.
"What's important is the fitness and the sharpness," Rooney told the club's official website.
"I'm happy with how I'm doing. I've been training for almost two weeks and I've been working hard. Everything's going well so far.
"I feel good. I played two games in the summer and I played against Valerenga.
"Everybody needs games to get up to match fitness - not just me. I'm training well, though, and I'm looking forward to the next few games.
"Hopefully, by the start of the season, I'll be ready to play."