Clayton Mortensen saw Red Sox starter Aaron Cook walk off the field in the second inning Saturday with a cut on his left knee and go into the clubhouse. Told he was going in to pitch, Mortensen jogged from the bullpen to the mound.

But once he arrived, Mortensen was told to go back. Cook had decided to stay in the game.

“That was weird,’’ Mortensen said. “They told me Cookie was going to try and pitch through it and be ready just in case.’’

Cook couldn’t get through the third inning, and Mortensen did come in at that point to face Mark Reynolds with two runners on and two outs.

Reynolds connected on the sixth pitch Mortensen threw, driving it over the wall in left for a three-run homer. That gave the Orioles an 8-0 lead in a game they went on to win, 8-2.

“I tried to sneak a fastball in on him and left it too much over the plate and he put a good swing on it,’’ Mortensen said. “He’s a dangerous hitter. It was a horrible pitch by me.’’

Mortensen allowed two hits over 3 ⅓ innings and struck out five without a walk. In two appearances since being called up, he has allowed one run in 6 ⅓ innings and struck out 11.

“It’s worked out OK,’’ Mortensen said. “I’ve been able to keep them from getting squared up.’’

Scott Atchison also pitched well, throwing three near-perfect innings. He retired nine of the 10 batters he faced, allowing one walk. Atchison has a 1.50 earned run average in 12 appearances.

“Just trying to throw strikes, get outs, and go from there,’’ Atchison said. “Still early in the year, got a long way to go. But I feel good right now.’’

After using six relievers for eight innings Friday night, Mortensen and Atchison helped save the bullpen for Sunday’s series finale.

After a rough start to the season, the Sox bullpen has given up only six earned runs in its last 41 ⅓ innings. That is partly attributable to the relievers settling into roles.