Diamond appeared before a Rugby Football Union disciplinary panel in Brighouse tonight on two charges of misconduct arising out his side's 29-21 Aviva Premiership win over the Saints on September 23.
He pleaded guilty to pushing Johnston during a touchline incident and to making inappropriate comments in a TV interview after that game.
The panel imposed a 12-week suspension from the field of play and technical area and from communicating directly with match officials during the game but suspended the ban until May 31, 2013.
Diamond was also fined a total of £5,000, £1,000 payable immediately with the remainder also suspended until May 31, 2013.
Both suspensions are on condition that there is no repetition of misconduct and, in addition, Diamond was told to receive a course of media training before the end of the season.
Sale have already carried out an internal investigation into the incident, which occurred on the touchline during the first half of the game, and say they imposed an unspecified sanction on Diamond.
Sale trailed 18-9 shortly after half-time in the match but came back to win, with Northampton at one stage reduced to 13 men after captain Phil Dowson and flanker Calum Clark were sent to the sin-bin.
Meanwhile, Newcastle have lost an appeal against a four-week suspension handed to James Hudson for a dangerous tackle in a game at London Irish on September 24.
Hudson is suspended until November 1.
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