Willemse received a red card in the 58th minute of last Friday's URC clash at Cape Town Stadium for the challenge, which was more clumsy than malicious. His shoulder made contact with Hamer-Webb's head as he pivoted to shift his attention to the attacking player alongside him.
The 27-year-old reportedly accepted a guilty charge to receive a reduced sanction, even though the Stormers felt they had a case to argue.
He was handed a six-match suspension, which was halved due to mitigating factors. An additional match was scrapped on the condition that he attend a "tackle school" programme.
As a result, Willemse will miss the Stormers' URC quarter-final against Glasgow on 30 May and the semi-final the following week, should the team progress.
If he had pleaded not guilty and failed to argue his case, he might have copped a full six-match ban, which would have resulted in him missing the start of the international season for the Springboks.
Despite the more lenient sentence, it's a big blow to the Stormers' URC title aspirations, as they will be without one of their most decorated backline players as they try to play their way into the final.