PHOENIX -- When the Cubs acquired Alex Avila from the Tigers at the non-waiver Trade Deadline, Chicago manager Joe Maddon made a point of saying the catcher was not a backup. Little did the Cubs know Avila would be pressed into regular duty this season because of an injury to Willson Contreras.
Contreras, last week's National League Player of the Week, underwent an MRI on Thursday in Phoenix on his right hamstring, which he injured running to first base in San Francisco on Wednesday. The Cubs are not expected to announce results of the test until Friday.
Contreras maintained a positive attitude despite the injury, which could sideline him for a few weeks. He posted on Twitter: "On my way! To get the MRI. It's time to get healthy and come back stronger than before. Thank you all for the support."

Contreras, 25, was leading the Cubs in the second half with a .311 batting average, 10 home runs and 29 RBIs. He's been such a key piece to the Cubs' offense that Maddon has been finding ways to keep him in the lineup, inserting Contreras in left field on Saturday and at first base on Wednesday. In 31 games in the cleanup spot, Contreras was batting .305 with 13 home runs and 35 RBIs. He's second on the team to Anthony Rizzo with 70 RBIs.
"He's been carrying us, really, the last couple weeks," Cubs pitcher Kyle Hendricks said of Contreras.
Avila has also been impressed by Contreras.
"He understands himself and knows his swing, and it doesn't seem like he gets tired," Avila said. "It's nice being that age. He's got tremendous ability and has an idea how to use it."

Asked if he was comparable when younger, Avila shook his head.
"I've never had that much energy," Avila said.
Avila will need it now. With Contreras out, the Cubs will call up catcher Victor Caratini from Triple-A Iowa, but Avila will be expected to handle the majority of the catching. The real test will come on Saturday when Jonathan Lester starts. David Ross, who was Lester's personal catcher, has no plans to come out of retirement to fill in for Contreras behind the plate.
"This guy's not a backup," Maddon said when the Cubs acquired Avila. "You look at what he's done this year and look at his numbers and his history, and he's young enough to play more often."
The Cubs had been looking for an experienced catcher to back up Contreras since parting ways with Miguel Montero in late June. Avila, 30, was batting .274 in 77 games with the Tigers.
The Cubs had a 1 1/2-game lead over the surging Cardinals and the Brewers in the NL Central on Thursday, an off-day, before opening a three-game series against the D-backs on Friday.
Earlier this season, the Cubs had to deal with injuries to Jason Heyward, Benjamin Zobrist and Hendricks. Contreras, though, may be tougher to replace.
"They know he's not here right now," Maddon said of the Cubs' players. "They know how important he's been to us. And now other guys realize, 'Hey, let's go. We have to pick up the slack that has been created.' I've seen it before."