What would be likely to happen if a large excess amount of phosphorylated Rb is injected into cells during the G1 phase

it would block the S-phase entry and cell growth

Explanation:

The Rb protein is known to be a major G1 checkpoint. Rb is also known to be a tumor suppressor since this protein limits cell proliferation by preventing entry into the S-phase entry and cell growth. When the cell is ready to enter the division phase, Rb is phosphorylated and thus inactivated. In HL-60 cells, it has been shown that the levels of phosphorylated Rb protein increase with progression from G1 to S and to G2/M. This protein acts as a negative regulator that blocks the activity of the E2F transcription factor.