Mr Strain is widely seen as a successor to CEO David Potts once the latter decides his turnaround programme is complete.
Mr Strain will assume the additional responsibilities of group commercial director with immediate effect. His new role is group chief finance & commercial officer.
He takes over the commercial director role from Darren Blackhurst, who is moving to a projects role reporting to the Mr Potts.
Analysts welcomed the move and said this is a David Potts-led change.
One analyst said: "Darren Blackhurst has done a terrific job and he's been in the role for three or four years.
"Now is the right time for Trevor Strain to take the job on. Mr Strain has been getting more involved in the commercial side and he has always had more than a financial remit. He is a big beast in the business."
Mr Blackhurst will be taking on projects like direct sourcing, which is a big issue for Morrisons. Mr Blackhurst has a lot of Asian experience as Morrisons looks to direct source more from Asia in areas like clothing. With Brexit coming up and the CMA investigation into the Sainsbury's/Asda merger, Morrisons believes there is lot to be done on direct sourcing.
Mr Potts said: “Trevor has played, and continues to play, a critical part in the ongoing turnaround of Morrisons and his responsibilities are already significantly wider than those of the traditional CFO, including our property and online operations. Now is the right time for him to take on the important new responsibilities of commercial director, where he has already been increasingly involved.
“Trevor has built an outstanding finance team, with significant strength in depth, providing the capacity required for his new responsibilities.
“We are now in our fourth year of the Morrisons turnaround and I would like to thank Darren for the contribution he has made as commercial director during that period. He joined at the start of a critical period for Morrisons, and has played a key role in improving the shopping trip for customers and developing our relationship with suppliers.”
Analyst Clive Black at Shore Capital said: "We see this change, and the concomitant adjustments to management reporting lines, as the ongoing evolution of Morrisons' human capital, embracing the development of capability and broad succession.
"Morrisons has been a very successful turnaround story, with Messrs Blackhurst & Strain key cogs in the delivery engine, and we believe the future remains bright for this fixing, broadening and growing food maker and shopkeeper."