When Yamaha all eyes were on Jorge Lorenzo and his reaction to having his arch rival back in the team.
Rossi and Lorenzo have brought Yamaha six world titles between them, but their three-year term as team-mates between 2008 and 2010 caused plenty of headaches for the Japanese factory team, too.
Rossi and his crew were widely credited for the turnaround in fortunes at Yamaha, transforming the struggling YZR-M1 machine into a championship-winning bike from the off in 2004.
Two world titles and 29 race wins in his first four years elevated the Italian to legendary status and, despite not winning the title in 2006 or 2007, made him untouchable in the team.
Then Lorenzo arrived. The double 250cc world champion from Spain came straight into the Yamaha factory team as a rookie in 2008 and immediately lay down the gauntlet, with two podiums and a win in his first three races.
That Rossi's newest challenge should be a rookie threatening to beat him on his own bike understandably did not go down well with the then five-time MotoGP champion.
Over the next three years, the relationship deteriorated to the point that a wall was erected between the two garages to stop data sharing. Then Rossi's "it's me or Lorenzo" ultimatum to Yamaha ended with the Italian's departure to Ducati.
As any rider will tell you, a team-mate is the first person to beat. If that colleague also happens to be your biggest rival on the track, then all the ingredients are there for close and spectacular racing.
Coming into the 2009 Catalunya race, Lorenzo was ahead of Rossi in the championship and there was all to play for.
The epic last laps of the race felt like they were about more than the win and the championship, though. It felt like an on-track outpouring of the off-track battle to be number one at Yamaha.
After a thrilling last lap, Rossi won Lorenzo's home race with an audacious overtake on the final corner. He never lost the championship lead again that season, taking his seventh premier-class title at the end of it.
Rossi may then have thought he was in a strong position to deliver the "him or me" threat to Yamaha. But ultimately it was Lorenzo who won the battle to be Yamaha's top rider.
When Yamaha clearly refused to give up on their newest talent, Rossi carried through his threat, signing a big money move in the middle of 2010 to ride for Ducati for the subsequent two years.
The Motegi race in October 2010 came just days after Lorenzo, who was cruising to his first premier-class title, had penned a new two-year contract with Yamaha.
It was to be another thrilling race between the team-mates, but Motegi was everything Catalunya was not. Where Catalunya was a clean fight between two riders desperate for the win and the championship lead, Motegi was a scrappy - and at times almost dangerous - clash for third place, which Rossi clinched.
Neither rider needed the extra three points that finishing third would have given them. In fact, there was everything to lose for Lorenzo given the title was all but wrapped up.
But anyone who had watched the Rossi-Lorenzo saga unfold over the previous three seasons could see that this was a battle mostly about pride. For the fans, it was a great piece of drama. For the riders, the parting of ways could not come soon enough.
Rossi now returns to a team where world champion Lorenzo is the number one rider and the spearhead of development of the M1 bike.
With the new season starting at the Losail circuit in Qatar this week, how will the relationship fare second time round?
"They are in a completely different moment in their career," Yamaha team manager Wilco Zeelenberg told BBC Sport.
"Three years ago, there was quite some tension there, but at the moment they accept each other very well. Vale is still very fast and Jorge realises that he's a nine-time world champion, so he has to beat him.
"Before, Jorge was the upcoming young guy, but he's become a two-time world champion in MotoGP. Valentino also accepts that Lorenzo is a fast rider and a world champion. That's a big difference compared to 2010."
Even Yamaha might have been surprised by how well, publicly at least, Rossi and Lorenzo appear to be getting on. The relationship throughout winter testing, while still far from friendly, does seem to be built around a healthy, mutual respect.
But will that last once the lights go green or could there be a repeat of that famous clash in Motegi?
"I think there are no big differences between Valentino and Jorge in riding style," says Zeelenberg.
"Sometimes that clashes because Jorge is very smooth and fast mid-corner and Valentino is very strong in the braking area and in blocking and doing last moments in last laps. That can create problems, but I think now they are both mature enough to see that it's important to keep their heads looking forward. But, of course, they both want to win."
However the relationship pans out off-track this year, on the track they are two very talented riders on equal machinery, with a lot of history and each with plenty of motivation to beat the other and prove, once and for all, that they are still the number one.
As far as racing rivalries go, it does not get much better than that.