SAN DIEGO -- Clayton Richard's recent struggles cost him his place in the Padres' starting rotation. From there, his balky knees spelled the end of his season.
On Tuesday, Richard was placed on the disabled list with inflammation in his left knee. Later in the day, manager Andy Green revealed that Richard will undergo surgery on that knee, beginning a recovery process that will last at least six weeks and end Richard's season.
Richard proved a valuable innings-eater for the Padres during the first three months, posting a 4.29 ERA through the end of June. He hasn't won a game since and struggled mightily in those 10 starts, with a 7.49 mark.
Meanwhile, left-hander Eric Lauer is slated to return from a left forearm strain later this week. As they continue to give opportunity to their young starters, the Padres decided Lauer would take Richard's place.
"We made the decision, outside of this, to go ahead with the rotation change," Green said. "Then, once we made that decision, everybody felt, top to bottom, Clayton included, that it was in his best interest and our best interest to go ahead and have him start that process of rehab."

At the end of the 2017 season, the Padres signed Richard to a two-year contract extension. He earned the nod on Opening Day and posted a 5.33 ERA in 27 starts this year. If all goes according to plan, the veteran left-hander will be ready for the beginning of camp next spring, when he'll presumably compete for a roster place -- whether in the bullpen or the rotation.
As for the current starting five, Lauer's return gives the Padres four rookies, with Robbie Erlin as the lone exception.
Right-hander Bryan Mitchell is also slated to return from his right-elbow impingement -- likely on Saturday when rosters expand. The Padres haven't yet determined whether he'll slot into the rotation or a bullpen role. Fellow righty Luis Perdomo (right shoulder strain) is a bit further back, but the same holds true.
The Padres have five off days during the season's final four weeks, making it unlikely they use a six-man rotation for any extended period of time.