(Reuters) - World 100 meters champion Justin Gatlin needs to put behind him the disappointment of a coaching change before he can move forward with preparations for the 2018 season, his new trainer said.

Justin Gatlin of the U.S. kneels down in front of Usain Bolt of Jamaica after the final of the Men's 100 Metres final during the World Athletics Championships at London Stadium, August 5. REUTERS/Phil Noble

The American sprinter sacked coach Dennis Mitchell in December after doping allegations were made against Mitchell, who like Gatlin was previously banned for doping.

Gatlin is now back with respected former U.S. Olympic and national team coach Brooks Johnson, who trained the sprinter in 2010-11.

“We have not gotten into that kind of intense training but my sense is the thing that needs attention and needs to get worked through is basically his disappointment and upsetment with having to make the changes and the conditions under which he had to make them,” Johnson told Reuters from their training base in Orlando, Florida.

The change occurred after Britain’s Daily Telegraph newspaper set up a sting where the publication claimed Mitchell, a former medal-winning sprinter, and sports agent Robert Wagner offered to supply performance-enhancing drugs to undercover reporters claiming to be making a movie featuring a doped athlete.

“I was shocked and surprised to learn that my coach would have anything to do with even the appearance of these current accusations,” Gatlin, who served two doping suspensions earlier in his career, said in firing Mitchell.

Now he is trying to rebuild with his former coach.

“It wasn’t easy,” Johnson said of the transition. “Justin is a very loyal person. I think having to make the move took a lot out of him.”

While anti-doping officials investigate the allegations, U.S. Anti-Doping Agency said that Gatlin was eligible to compete.

Johnson, before he took the job, said he also was assured by USADA and USA Track & Field that Gatlin was under no sanction or suspected sanction.

Their previous parting was amicable because Gatlin works best with talented training partners, which Johnson did not have at the time, the coach said.

His group now includes Olympic gold medalist LaShawn Merritt and Jamaicans Odean Skeen and Andrew Riley.

To make things easier, Gatlin is essentially doing the workouts he did in 2015 when the American had the five fastest 100 meters times in the world before narrowly being beaten by Usain Bolt at the world championships.

“I would like him so that mentally and spiritually, he feels comfortable in his new surroundings,” Johnson said.

“Once we get that happening and get him some quality work with some quality people, I think the results will be pretty much self-evident,” Johnson said.

Gatlin, in an interview before the newspaper story, told Reuters he wanted to return to 2015 form and perhaps take a shot at Tyson Gay’s American record of 9.69 seconds.

Gatlin, who beat Bolt for the 2017 world title, has a best of 9.74 set in 2015.

The American also wants to defend his world championship in 2019 and bow out at the 2020 Tokyo Olympics, 16 years after winning the Olympic gold medal in Athens.

“Justin is a warrior,” Johnson said. “So he is always looking for another mountain to climb.”

The longtime mentor scoffed at the idea Gatlin, who will be 36 next month, is too old to be a contender for the big prizes.

“I think the idea of age, aging and maturity and stuff like that is grossly stated and understood,” Johnson said.

“At 33 LeBron James is having his best year. Do you think... that his body has taken less of a beating than Justin’s?”