German rider Fritz will take over the Motoxracing squad's solo Yamaha R1 as the team's regular rider Tamburini fractured his right foot during training ahead of the Portuguese round on May 20-22.
Fritz made two wildcard outings last year on an EWC-spec Yamaha for the YART team at Most and Jerez, scoring a best finish of 10th.
Estoril will therefore mark the 29-year-old's first outing on a proper WSBK-spec machine.
Fritz commented: “When I got the news that I’d be racing at Estoril with Team Motoxracing, I couldn’t believe it!
"I was mentally preparing for a wildcard entry at Most at the end of July, something I did last year too, but knowing that I can race in the championship right away makes me so happy!
"I’m sorry for Roberto and hope he gets well soon. I can’t wait to get on the Motoxracing R1; this is a great opportunity for me, and I’ll do my best to make the most of it. I want to thank [team boss] Sandro Carusi for trusting in me – I’ll try to repay him with the best possible results.”
Tamburini was himself a last-minute replacement at Motoxracing for Isaac Vinales, who was named on the initial season entry list but parted ways with team ahead of the opening round at Aragon last month.
The Italian rookie has so far scored a best finish of 11th in the second full-length race last time out at Assen.
“We are sorry about Tamburini’s injury and hope he will recover very soon," said Carusi. "Both he and the team were making constant progress and it’s a real shame that this injury will prevent him from continuing along that path.
"I’m pleased to welcome Marvin Fritz, who we know is an experienced rider who has already competed in WorldSBK and who knows the R1 well, as he also uses it in world endurance, albeit in a different configuration. We’ll do our best to make him comfortable as part of the team.”
