France forward Martial missed almost two months of action with a thigh injury before returning off the bench late in the 1-1 draw against Liverpool on October 20.
The Red Devils have since won five of its six matches since, with a limp 1-0 loss at Bournemouth the only blot in that run.
Martial, James and Marcus Rashford were in scintillating form as Brighton and Hove Albion were swept aside 3-1 on Monday (AEDT), a result that left United seventh heading into the international break.
And James, who turned 22 on Sunday, believes Martial's influence has been pivotal to the recent improvement.
"Since he's come back in, we've got that focal point," James said after the Brighton game.
"He's a great link-up player. He can come short or go in behind. It's been important for me and for Marcus since he's come back into the team to get that interlink.
"Since he's come back, he's got goals and he's got assists and he's been great for us."
James arrived from Championship side Swansea City in a reported £15million deal in June and has taken on a prominent role in the team, appearing in all 12 Premier League fixtures this term.
The Wales winger concedes he did not expect to feature so much but is relishing the opportunity.
"Coming in was always going to be tough and it was going to be different from playing in the Championship," he added.
"I think I've taken it in my stride, tried to calm myself down a bit and think, 'I'm here for a reason'.
"At the start of the season, I maybe didn't think I would play as much. Every game I've been learning and the gaffer has trusted me and I've really enjoyed it."